Not sure if this has been suggested before but Ehlers danlos! It is a chronic connective tissue disorder that can cause hyper-mobility, stretchy or silky skin, and can be fatal depending on the type.

Just a note before writing this: I have heavily researched this topic, and live with undiagnosed chronic pain of my own. That being said, if I misrepresent anything about EDS I would like to be corrected so I can edit the post accordingly. I am getting most of this information from the Ehlers Danlos society. Link: https://www.ehlers-danlos.com/eds-types/ and other similar websites, as well as conversations with my own doctor about EDS and hypermobility specifically.
there are also examples of EDS in felines and dogs!
“Lions with EDS in the wild wouldn’t live!!” they also wouldn’t live past being a cub with blindness or deafness in 90% of cases but those are not cub specific lethalities in game, but I respect your opinion if that’s why you don’t support ^^

I’m suggesting a type called Hypermobile EDS, which is a type that causes varying hypermobility to the joints and especially overextension in the wrist, elbow, and knees. The lions with this could have a lowered stat rate, lower hunting/patrolling success, but a higher chance at succeeding in cub training; as pain from EDS nearly always starts in the teenage stages of life, children with EDS typically have hypermobility without the pain (though they CAN injure themselves easily, especially by popping a bone out of place or similar injuries to the joints) They could be at a higher risk of dying without a broodmother
Ehlers danlos also famously makes peoples (and animals) skin more elastic, especially in felines. They are much more prone to skin tears and bruises, and have incredibly stretchy or loose skin.
The muzzle would also be affected, as individuals with EDS have a thinner palate, the muzzle could possibly look thinner than normal, with loose skin around the cheeks.
The legs in the lineart could also show the overextension in the joints by giving their front and back legs a “double jointed” appearance, think when somebody rotates their elbows at will.
EDS will also make the lion much for fatigued than normal- again affecting their hunting, patrolling, and possibly them being kinged. If one is able to be kinged, there could possibly be a new personality added, or one specific to EDS lions, that may be the opposite of Hyperactive. Not sure about what the pros or cons of the personality would be or what it’d be called, but i’m 100% open to suggestions!
The pose for the affected lions could be them sitting down or resting in some way, with overextended joints being visible in the legs, sagged skin around the jaw, neck, possibly other areas of the body like the limbs, and paws positioned in an almost “unnatural” position to the legs, and the muzzle or mouth thinned slightly.


Flavor texts:
-Cub training
“(agility) [Cub] managed to slip inside of a small gap in the rocks! Just as you began to panic, you see them squeeze their way out in a way that seems almost impossible, and continue their training as if nothing happened.”

“(fighting) [Cub] got into a scrap with another littermate, and you hear a loud pop and see their arm twist! They run to their broodmother in tears, where she quickly fixes the issue. Ouch.”

“(fighting) [Cub] and their littermates decide to scrap over their favorite food! even though theyve won the fight, you can’t help wondering why they look so beat up.”

-normal flavor texts
“[cub] says they want to show you a cool party trick. Before you can answer, you watch as they twist their paws around to face their stomach. Their broodmother quickly tells them off, warning them they could dislocate the joint again. Again?”

“you see [Adolescent/adult] wading in a waterhole, before you can question them, you see the familiar wince on their face before finding a more comfortable spot, finally able to relax their joints.”

“[adol/adult] seems more exhausted today than normal. You remember they went on an especially difficult hunt, and get them some fermented marulas to snack on. They deserve the rest.”

“[Adol/adult] approaches you with a limp, asking you if you can pop the shoulder back into place. It’s a simple task, but you can’t help but to wonder how it happens so often.”

“[Adol/Adult] trots to you happily, rubbing up against you to groom. The extra skin on their face is a challenge, but you’re able to get each other squeaky clean in no time”

There’s definitely more than could be added, if you can think of any let me know! flavor text suggestions or other lineart change suggestions are also very welcome! i’m open to any criticism or ideas about this ^^