I've seen quite a lot of players become confused on various parts of breeding items, so I figured I'd throw a guide out to hopefully help! (If anything seems missed or not elaborated enough, give me a shout! It should be pretty thorough, but it's possible that I've missed something, given the sheer amount of breeding mechanics and items in game.)

Which items/passable mutations roll first? Well, here's a list!
Leopon/Overgrown Fur/Tigon > Patches/Piebald > GMO Cow/Lion Meat > Cotton Root Bark > Fertility/Natural Chance > Lion Scrotum > Primal Variations > Mukombero

GMO Cow (common abbreviation - GMO)
A usually high chance of throwing a mutation, usually a common one, but nothing is ever guaranteed! Can be combined with Cotton Root Bark, and if both trigger at the same time it could possibly make the mutation more rare. DO NOT breed with a GMO and then use a CRB on your lioness! You will waste the GMO effects! CRB causes a miscarriage, and therefore the GMO litter will be lost. Cannot be combined with Lion Meat or Lion Scrotum. (This item can be obtained in September in the event shop, for 150 Jewel Beetles!)

Lion Meat (common abbreviation - LM)
A mutation breeding item which has a higher chance of throwing a mutation than a CRB, but a lower chance than a GMO. Can also be combined with CRB for the same possible effect, however again as with the GMO you want to use this item on your lioness AFTER you use the CRB. Cannot be combined with GMO or Lion Scrotum. (This item can be obtained in the January event shop, for 2,000 Silver Beetles, or in the Gorilla Enclave for 300 Monkey Teeth!)

Cotton Root Bark (common abbreviation - CRB)
A mutation breeding item which induces a miscarriage in the pregnant lioness it is used on. A lower chance to activate than a GMO or Lion Meat, however user-base studies have indicated that it may have a possible trigger rate of about 15%. High suggestion of not using this item on lionesses which have a mutation that can pass (leopon, overgrown fur, all piebalds), as you may be aborting a cub with the mutation! (This item can be obtained in the Oasis, for 1 Golden Beetle!)

Lion Scrotum (common abbreviation - lion scrote)
Has a 25% chance of passing the mother's mutation to a cub. This chance rolls on each cub, so you're more likely to hit it the more cubs in the litter you have.This item does not work on hybrids - tigons, unreleased, and leopons - or primal/felis, however does work on OG fur and piebald! Cannot be combined with GMO or Lion Meat. BEWARE DU BREEDERS this item only has a 5% chance of passing on the Double Uterus mutation! (This item can be obtained in the February event shop for 80 Heart Shells, or in the May event shop for 400 Manticora Beetles!)

Crunchy Worm (common abbreviation - crunchy)
Has a 25% chance to pass the entirety of the mother's looks to one cub in the litter. This will not trigger on more than one cub, so you aren't more likely to trigger it whether you have one cub or four. This item will not pass on the mother's mutation. It can be used with items such as Ochre Gnawrock and Ochre Saltlick, however if Crunchy Worm triggers it will override the other item and the marking or mane will not pass to the cub unless the mother has it and it passes from the Crunchy Worm. GMO, Lion Meat, and CRB can be used with this item, but remember to use CRB before Crunchy Worm! (This item can be obtained in the October Manticora shop, for 300 Blood Beetles! Note that you must stay with Nirah in order to keep the Manticora Shop!)
*Please keep in mind that First Gen Leopon overrides everything, and if you happen to get a First Gen Gnawrocks and Crunchy Worms won't be able to affect that cub.

Ochre Gnawrock (common abbreviation - Gnawrock
This item passes down a chosen marking from a lion or lioness with 100% certainty! The cub(s) will inherit the marking at the parent's opacity, and will trigger on the entire litter. You can use one on each parent and the cubs will inherit both markings! Can be combined with all other breeding items, but remember to use CRB before Gnawrocks! (This item can be obtained in September in the event shop, for 150 Jewel Beetles!)
*Please keep in mind that First Gen Leopon overrides everything, and if you happen to get a First Gen Gnawrocks and Crunchy Worms won't be able to affect that cub.

Ochre Saltlick (common abbreviation - Saltlick)
This item passes the entirety of the chosen parent's mane to the entire litter of cubs! Can be combined with all other breeding items, but remember to use CRB before Gnawrocks! If given to BOTH parents, the cubs can inherit any combination of the parents' manes. If mom is Savage Ebony and dad is Royal Orchid, the cubs can be Royal Orchid, Savage Orchid, Royal Ebony, or Savage Ebony. (This item can be obtained in September in the event shop, for 100 Jewel Beetles!)

Pennyroyal
This item can be fed to a Primal, Felis, or Smilus king to increase the chances of a Smilus or Felis being born! This does not increase the chances of receiving a mutation, however if a cub rolls for a mutation it is more likely to be one of the variations.
** This item can be used on Ferus kings, HOWEVER will only give a boost when a Ferus female is bred to a Ferus male. (This item can be obtained in the Oasis for 1 Golden Beetle!)

Mukombero
This item is fed to a lioness and will give one Primal (Ferus) cub in her litter. The stud doesn't have to be any primal variant, a cub in the litter will be Ferus regardless of what the father is! This item must be used on a non-mutated lioness at least 2 years of age but no more than 14 years of age. The lioness can ONLY be bred at night, Lioden time, nor real time, while this item is in use! (This item can be obtained through a special explore encounter during March.)
* Note that while this item rolls last, one cub will be overwritten if it has rolled a mutation already.
HERE is the item's official news post, as it goes more in depth on Ferus lions.

Buffalo Scrotum (Common abbreviations - buff(y) balls, buff(y) scrote)
This item guarantees at least 3 cubs in a litter! This item is consumed by the stud. (This item can be obtained in the February event shop for 60 Heart Shells, or in the August event shop for 600 Dry Bones!)

Grain of Paradise (Common abbreviations - GOP, grain)
This item guarantees at least 2 cubs in a litter! This item is consumed by the stud. (This item can be obtained in the February event shop for 30 Heart Shells, or in the August event shop for 300 Dry Bones!)

Yohimbe Bark (common abbreviations - bark, y.bark)
This item removes one day of breeding cooldown from your lioness! You may use as many of these on your lioness as you want, however she must not be nursing! You may kill, chase, nature reserve, or age the cubs in order to use these items before your lioness "should" be done nursing. (This item can be obtained in the February event shop for 30 Heart Shells, or in the Gorilla Enclave for 250 Monkey Teeth!)

Shadow of Death (common abbreviation - SoD)
This item ages your lioness by one month per SoD used, reducing her breeding cooldown by one month as well! This item cannot be used on lionesses under 2 years old, but can be used even if the lioness is nursing or pregnant. You can use this to insta-birth cubs though I'm not sure why you would use them for that purpose, as Instant Cub Deliveries are far cheaper. (This item can be obtained in the October event in the Hell Shop for 30 Nepheline Feathers.)

Black Stallion
This item ensures that your lioness will fall pregnant in a singular breeding attempt during the breeding it is used in. It will not affect future breedings, and you'll have to use black stallions on those breedings as well if you want the effect! Cannot be used on Infertile lionesses. (This item can be obtained through the Slots minigame as a High tier prize or in the Serengeti Shuffle minigame as a prize in the 3500-4444+ prize tier, as well as a purchasable item in the Harbinger Shop during September for 15 Eldritch Tokens.)

Hyena Meat
This item permanently lowers your lioness' fertility to 1%! (This item is obtained at the end of the January storyline!)

Angelic Blessing
This item works exactly like a Black Stallion, but it works for your lioness' entire life, on every breeding. (This item can be obtained during the October event in the Heaven Shop for 60 Demonic Scales.)

Shamanic Stew
This item is used on the stud and boosts the resulting cubs' strength. (This item can be obtained in the November event for 80 Scarab Beetles.)

Roasted Vulture
This item is exactly like the Shamanic Stew, but for speed! (This item can be obtained in the January event in the Carrion Shop for 500 Silver Beetles.)

Rock Salt
This item guarantees the passing of the parent's skin color! It can only be used on one parent per breeding, as you can't have two skin colors passing at the same time. It can be used on either the stud or lioness. If given to both parents, only the mother's skin will be passed to the cubs. (This item can be obtained in Monkey Business for 200 Silver Beetles!)

Opal Saltlick
This item guarantees the passing of the parent's eye color! It can only be used on one parent per breeding, as you can't have two eye colors passing at the same time. It can be used on either the stud or lioness. If given to both parents, only the mother's skin will be passed to the cubs. (This item can be obtained in Monkey Business for 500 Silver Beetles!)

Instant Cub Delivery (common abbreviation - IBF)
This item can be used on a pregnant lioness to cause her to give birth immediately! Note that you must nest your lioness before using this item, however no worries if you forget! The game will give you a popup to remind you if you try to use the IBF before nesting. (This item can be obtained through the Oasis for 2 Golden Beetles!)

Zebra Heart
This item will force your lioness into heat! However, she CANNOT be on a breeding cooldown! (This item can be obtained through the Oasis for 2 Golden Beetles!)

Cape Bulrush (common abbreviation - bulrush)
This item can be used on your own king to add 3 extra studding slots! You may use up to two of these items per week. (This item can be obtained through the Oasis for 3 Golden Beetles!)

Vuka Vuka
This item allows you to change the fertility category of your lioness. You may NOT select Goddess or Infertile, and the exact percentage is entirely random. (This item can be obtained through the Oasis for 3 Golden Beetles!)

Chasteberry
This item raises your lioness' fertility by 10% temporarily. The effects wear off once she's bred, and you'll have to use them again the next time you breed her if you wish to up her fertility again! You can use up to 5 of these on a lioness at a time. (This item can be obtained through Quests & Snakes in Crossroads for 100 Silver Beetles!)

Tackweed
This item lowers your lioness' fertility by 10% temporarily. The effects wear off once she's bred, and you'll have to use them again the next time you breed her if you wish to lower her fertility again! You can use up to 5 of these on a lioness at a time. (This item can be obtained through Quests & Snakes in Crossroads for 60 Silver Beetles!)


Great Tit
This item is used on the king during breeding and ensures that the entire litter will be female. This is helpful for breeding female specific mutations such as mane mutations and double uterus.


Red Cock
This item is used on the king during breeding and ensures that the entire litter will be male. This is helpful for breeding male specific mutations such as Primal (Smilus).

Passable Mutations and Pass Rates
The following are currently the ONLY passable mutations in the game!

Leopon - First Generation pons have a 10% pass rate of their mutation whilst every other generation has a 5% pass rate.

Tigon - First Generation gons have a 10% pass rate of their mutation whilst every other generation has a 5% pass rate.

Piebald/Patches - All piebalds/patches can pass from both males and females and are believed to have a 5% pass rate. *This mutation cannot be gotten randomly and must either be applied with a Piebald or Patches Mutie On Demand, or from a Piebald/Patches parent, and a Piebald/Patches can only pass down the piebald that they have, not a different piebald/patches variant.

Primal - Normal Primal kings can pass their mutation and are believed to have a 5% pass rate. *This mutation cannot be gotten randomly and must either be applied with MOD: Primal (it may ONLY be applied to male lions) or from a Primal father

Overgrown Fur - OG Fur females can pass their adult mutation to their female cubs, and are believed to have a 5% pass rate. *Make sure to check all cubs from an OG Fur mom at the Falcon Eye to see if they have the mutation! This mutation cannot be gotten randomly and must either be applied with MOD: OG Fur or from an OG fur mother. Cannot be passed to male cubs.

Primal (Smilus) - This mutation can be bred from a Primal, Primal (Felis), or Primal (Smilus) father and can only be passed to male cubs. The chances of a cub getting this mutation are unknown, however the chances can be boosted with a Pennyroyal.

Primal (Felis) - This mutation can be bred from a Primal, Primal (Smilus), or Primal (Felis) father. This mutation can be passed down to both male and female cubs, but remember that only the males can pass their mutation! The chances of a cub getting this mutation are unknown, however the chances can be boosted with a Pennyroyal.
*Neither a Smilus nor a Felis father is more likely to pass their mutation than a normal Primal! A Felis cannot produce a Smilus.

Primal (Ferus)- This mutation can be passed down from a Ferus parent or can be bred when a Mukombero is given to a lioness before her breeding. This mutation cannot be passed from Primals, Felis, or Smilus lions and can ONLY be bred from itself. This mutation can, however, pass ALL Primal variations, as well as receives a boost while fighting primates in Explore. Pennyroyals are only of use when breeding a Ferus female to a Ferus male.

Dorsal Fur - This mutation can be passed down from both males and females. Breeding two Dorsal Fur lions together doubles the chance of producing Dorsal Fur offspring. ALL lions with this mutation will be born with the Crested mane type.

Special Mention Mutations
Double Uterus - This mutation causes the lioness' litter size to double. You can breed her with two different studs, but be aware that the second stud's genes will simply overwrite half of the other stud's cubs. Meaning, if you breed your lioness to a Sunrise stud and then to a Sunset stud, you may be overwriting a Sunrise base. No matter which option you choose, the litter size WILL be doubled! So, you have a maximum of 8 cubs and a minimum of 2. You can use Buffalo Scrotums and Grains of Paradise to guarantee more cubs! If you choose to use GMO Cows or other items, such as an Ochre Gnawrock, I highly suggest NOT using a second stud, as you'll have to use a second round of those items if you want them to work on the second stud's cubs as well.

Primal Fangs - Primal Fangs is a fail mutation that can be bred from MALE Primal, Primal (Felis) and Primal (Smilus), as well as BOTH male and female Primal (Ferus) parents. This means you can stud a Primal (Ferus) female to any stud of your choosing, Primal or not, and still have a change at breeding Primal Fangs cubs. This mutation DOES NOT pass naturally but may be passed with the use of a Lion Scrotum at the normal rate of 25% roll per cub.

Mane Imperial - This mutation can only affect lions who are both female AND have the Royal mane type. If you are attempting to pass this mutation with a Lion Scrotum, it is heavily suggested that you use both an Ochre Saltlick to pass on the Royal mane as well as a Great Tit to ensure that the litter will be female, to increase your odds at the cubs inheriting the mutation.