This could be a lethal/non-lethal mutation that requires NO new art ( unless i’m mistaken )
This affects bleeding and the way blood clots, blood can either not clot or it can not clot properly/efficiently
The cub could survive a full healthy life however it’s chance of death is high.
The birth message could be
“ You lioness lays with her cubs as they suckle, As she pulls away she notices her cubs little gums bleeding, She’s concerned however decides it’s nothing to worry about “
Month 1: Your Lioness watches carefully over her cub scolding him when he gets too rough, she runs over him/her when he/she gets a scrape and shoos away the others
Month 2: Your lioness still keeps a watchful eye on her cub seeming to almost seclude him from the others
Month 3: You lioness looks concerned, she tells you that her cubs mouth bleeds when he/she tries eating meat, she returns to watching the cub carefully
Month 4: The cub sits off on the side swatting at a butterfly, she/he has been disallowed play with others
Month 5: The cub walks carefully eats carfully and does everything as slow and carefully as possible, they’re quite timid
Month 6: The cub has been weaned but only seems to eat small meat chunks

If they survive females will not be allowed to hunt but will be able to give birth
The cubs can get the same disease as her and she will most likely die during the birth due to bleeding out.

Males cannot spar, patrol, or become a heir. They can live a full live till natural death just as a female though it is very rare

Death message : “ (X) Was recently injured, a wound that might put some hurt on other lions seemed to be much more devastating as the lion/lioness slowly bled out due to its lethal mutation “

Give suggestions and i will change/edit my idea if i like it, please give me a reason why you support or do not support as it can help me to tweak this better.

Herb Alfalfa can be bought in oasis for 300SB for 3 uses, each use can drastically increase the lions survival chance PER day ( Alfalfa is found to help with clotting )

Cons:
Lions teeter between lethal and non lethal
Herb could become too pricy and thus cause lethality to go up

Neutral
No extra art

Positive
With each litter female has a 1/15 chance to have aCRB like miscarriage, her body would be trying it’s best to save her life, in turn if she her next pregnancy will act as if she had eaten CRB
“ But this will make muties too common!! “ And I understand but living to a birth age is rare for them and even wil alfalfa they have a pretty rough time living, Also a female having a cub? incredibly difficult for her and could cause death, Yes alfalfa can help but in the wild it honestly comes to luck.

Also females have a slightly higher chance of mutation and will never be above “ Good “ in fertility.
Again the counter to this is that these lions have a high chance to die before age 2, with alfalfa the chances drop to a reasonable survival, and with brood mother they can get to adolescent but from adolescent it all comes toalfalfa, at adult hood again alfalfa and if they make it to adulthood the females will most likely die during birth
This is a double edged sword, Yes she has a very decent chance of mutation but that’s if she even lives to her adult heat stage AND the birth.

To sum it up, she has a pretty decent chance of mutations however it’s hard for her to survive to the adult birthing stage

This could also just be an outright lethal mutations without all the mutation mess however i felt like i should add it as a possibility