While black (melanistic) servals do exist in the wild, I can't find any mention of a Black Serval as a character anywhere other than danbooru (I'm pretty sure she's not in any game, and she's definitely not in the anime). She only has three posts, and the costumes are not consistent. I think serval_(kemono_friends) + palette_swap or maybe just alternate_costume would probably be better.

Yeah, they're not the same exact costume so palette swap is out (same with player 2).

I'd go with alternate costume + alternate hair color for parent:3274589, and alternate costume + dark persona for post #3134017.

For all of those, there really isn't a tag for alternate animal ear color or alternate tail color, but there are tags for alternate animal ears and alternate tail, but I'm not sure those should be used.