So I had this filly born recently https://horseeden.com/horse.php?id=35702547 and her color shocked me a bit because she is technically just a black leopard. Honestly I didn't even know peacock existed in game, and I can't find anything about it in any of the color/pattern libraries. Is this genetic or is it like brindle and completely random? Also why is she almost varnish-esq or roanish instead of just solid black?

Peacock is not genetic. It's just a variation of Leopard. The roan like appearance comes from the peacock on its own. Peacock can be rerolled on and off so be careful

My guess is that she isn't black due to her appy variation. It appears to have taken over her base coat.