amelia is a mutation that causes one or more limbs to be missing from birth. since a lion may be able to survive with one missing, i suggest a mutation where two are gone, the back set.
this would be lethal and cause the cub to die at the 7 months rollover from lack of exercise.

CUB UPDATES:
birth: your lioness has given birth to (number of cubs)! to her shock, one of them appears to be missing its back legs! luckily, it is nursing normally, but youre concerned for its health.
1 month: (cub) is trying to follow its siblings, but falls behind a lot. it seems upset at this.
2 month: (cub) is gaining more weight than its siblings and likes to play alone. its mother is worried for its health.
3 month: yesterday, (cub) tripped while trying to walk around, and hurt its nose. now it won't leave its mother's side.
4 month: (cub) watches sadly while its siblings run around. you're trying to comfort it, but it doesn't want sympathy.
5 month: (cub) is still gaining weight and doesn't try to walk anymore. its mother grooms it sadly.
6 month: (cub) slept all of yesterday and today... its mother is already half-detached.
7 month rollover: (cub) died last night due to lack of exercise.

for art alteration, the cub would be laying down, supporting itself on its forelegs and looking apathetic