(Note: I'm aware that it's not a mutation. But it's the closest thing we have here in Lioden, it's a cat based disease so I decided to post it here. It has a different version that can affect dogs, so let me know if you would like me to revise it for Wolvden.)

FIP stands for Feline Infectious Peritonitis, which is a disease that is spread via a virus.
It has a high mortality rate, especially in kittens. It's very hard to treat and prevent.
FIP is mostly seen in kittens or cats with weak immune systems, as it goes for under-developed immunities.

There are two types:

Wet:
Persistent fever.
Lack of appetite.
Gradual weight loss.
Diarrhea
Swelling of the abdomen.
Fluid in the chest cavity.
Difficulty breathing.
Leaking Fluids.

Dry
Poor growth.
Anemia.
Jaundice.
Diarrhea.
Fever.
Eye inflammation.
Neurological problems.

In-Game Effects:
This mutation would most-likely be lethal. It's un-treatable. The lion would probably die at 5 months at the very most. More towards 3 months.

Appearance:
For the wet type the lion would appear bloated. It would have fluid coming out of it's nose, tear ducts, and even its mouth. Its legs would be slightly thinner.
For the dry type it would be less severe. Its skin and the whites of its eyes would be shaded yellow and it would be a lot thinner.

Interactions:

Wet:
Coming into the den:
*You heard [Cub] coughing as you walked into their den. You look at his mother with concern in your eyes, but you don't say anything.
*[Cub] hasn't been suckling properly lately. [Gender] said they were full after only eating a small amount.
*You try to can feel how hot [Cub] is as they sleep. Are they always like this?

Game notifications:
1 Month:
[Cub] looks a bit sickly today, [She/he] isn't suckling as strongly as the other cubs.
2 month:
[Cub] isn't gaining a lot of weight. Most of [his/her] time has been spent sleeping.
OR
[Cub] has died.
3 Months:
[Cub] isn't showing any interest in playing with the other cubs. [He/She] says their stomach is too full. They haven't eaten yet.....
OR
Cub dies.
4 Months(If they make it that long):
[Cub] has been laying on their back a lot recently. When asked why [He/She] says it's more comfortable for them that way.
OR
Cub dies.
5 Months:
Cub dies.

Dry:
Den:
*[Cub] tries to walk over, but they can't seem to find their footing. You come to them instead.
*[Cub] looked exited to see you, but when they came over they walked right passed you and started talking to a bush.

Game Notifications:
1 Months:
[Cub] looks different from all the other cubs. [He/she] seems a bit more......yellow.
2 Months:
[Cub] often tries to play with the other cubs, but they can't seem to walk right.
OR
Cub dies.
3 Months:
[Cub] ran into a wall after several attempts at walking. You and her mother are very worried about [her/him] getting hurt.
OR
Cub dies.
4 Months:
[Cub] looks different from all the other cubs, [she/he] is so small. You wonder why.
OR
Cub dies.
5 Months:
Cub dies.

That's all I have for now. If you don't support it please tell me why.

Sources:
PetMD