I have seen stuff recently about short spine syndrome, a rare genetic disease that shortens the spine of an animal. If you look it up, you can see animals with it, and they kinda look like fuzzy frogs.

I think this would be a cool idea for a mutation! It probably wouldn't be lethal, but maybe lions with short spine syndrome could have less success hunting or patrolling, and couldn't be kinged.

Also, because it is a genetic disease, it would have a small chance to naturally pass. Maybe they would also live less than the average lion because of their condition.

[edit:] alternatively, it could be a lethal mutation that lets them live until adolescent age, when they would die due to issues hunting and keeping up with the rest of the pride