This is a mutation that happens in cats. My own cat has it. This is purely cosmetic but I think it could be cool as a more common mutation on lions.

The front of the breastbone or sternum [this part is called the manubrium] sticks out a bit and causes a small protrusion on the front area of the chest, making it look more angular than on a normal animal and also causing a small lump that is visible on sleek haired animals.


[not my cat]

[also not my cat.]

for the lions, this would show as a small lump on the chest for the lioness and cubs, and on male lions, since the mane covers that area for all mane types, it would cause the mane to bulge outwards instead, being more visible on males than females.

It would have no gameplay effects and would show up from birth.

This is a small cosmetic defect so could possibly be one of the mutations caused by inbreeding. One males it would only alter the mane, on females/cubs the basic lineart.
It could possibly have good pose preference as that would show the defect best on a female.
Rough mockups here: