Appears?: At birth. Cub has a club foot (probably just one, front foot so it can be more easily seen. Maybe club foot right and club foot left could be two separate mutations?)
Affects gameplay?: Yes. A lioness cannot hunt, and a lion cannot patrol or become king. A cub may be able to train but have a high rate of failure (higher than normal). A lioness could still be a broodmother.
Lethal: No.
Alternate name: Talipes Equinovarus

Possibility:
A lioness is able to hunt, but she has a high failure rate alone and will drag the success rate of a trained team down. This doesn't mean she can't successfully bring in food, but the chances are far smaller.
A sub male can patrol, but instead of having 10 spaces available to him, he can only patrol 5 spaces.
A club foot male can become king, but he has a high strength penalty, and grabbing, pounding and kicking would fail more in battles.

It can happen to animals as well as humans. I have several favourites of a cat on flickr that has a twisted front paw (her name is Dizzy and she's very cute) that can ambulate mostly normally and do things cats can do.

NEW - Mockups (they're bad, but I'm not a great artist)
Club foot left/right

MUTATION SUGGESTIONS:
Spina Bifida [lethal] | [YOU ARE HERE] | Osteogenesis Imperfecta [lethal]
EYE SUGGESTIONS
Fruits for the Eyes | Assorted Eyes | Assorted Eyes: Part 2
MARKING SUGGESTIONS
November and Gem Markings | February and Gem Markings
Rosette Recolours | Giraffe and Inverted Giraffe
BASE SUGGESTIONS
Cake's Realistics | Cake's Semi-Realistics
Cake's Unrealistics | Cake's Combos
GENERAL SUGGESTIONS
Spoiler Button | Event Based Site Themes