Background

What are Teratomas?

A teratoma is a rare type of tumor that forms in a germ cell -- a cell that produces gametes (sex cells) --and usually appears in the lower body. The word teratoma literally means "monster tumor", not because they are exceptionally large, but because they produce different types of tissue "borrowed" from other parts of the body. Teratomas can include hair, teeth, and organ tissue such as thyroid or liver cells.

Teratomas in Animals

Teratomas are best-studied in humans, of course, but they have been found in many animal species as well. In domestic cats, it is relatively common to find teratomas in the uterus of an un-spayed female. There has also been at least one case of a visible teratoma in a big cat -- "Teratoma on Forehead of Puma Concolor". (this link contains images of facial deformities in cats that may be disturbing.) Although there has never been a report of a teratoma found in an African lion, there is nothing to suggest they could not develop one...

Teratoma In-Game

Art

A teratoma that is hidden within the body would not make for a very interesting mutation, visually. Therefore I suggest that the teratoma mutation on Lioden would feature a much more dramatic kind of teratoma, located across the face and neck, and include teeth, fur, and whisker tissues. The only art change would be for the adult stage, as the mutation is not visible before then. Teratomas would be compatible with all mane types and poses (it would not have a unique pose.)

Gameplay Effects

The teratoma mutation would not be visible until a lion reaches adulthood, but it can be detected by Falcon's Eye. It would affect both sexes equally and not be inherited/passed on to cubs.

Upon viewing a lion with a teratoma for the first time upon reaching adulthood, flavor text will display describing the mutation:
As you approach "name", you notice something strange on their head... it appears that teeth are growing right out of their face!
After this point, the lion may die randomly at any time up to the age of about four years, similar to the Deaf mutation. Unlike Deafness, lions with a teratoma can hunt and patrol like normal, however they cannot become kings.

Flavor Text

Because lions with teratoma can live for several years after the mutation appears, they would have unique flavor text like Deaf lions. This includes "health updates" and death messages.
If you have suggestions for relevant flavor text, please comment them below!

Conclusion

The teratoma mutation would be a unique, gameplay-changing lethal adult mutation. Right now, there is only the Deaf mutation as an adult lethal, but my vision is that more mutations will be added that affect an adult lion's life. Because teratomas are a very rare type of tumor, I believe they would work better as a mutation than as part of any illness/disease mechanic.

If you have any information I missed, suggestions/adjustments, or general opinions, please reply and tell me!