MONSTER GOAT DOES NOT MEAN UNDERTALE, its just a generic term for a reasoning of why he can turn into a big reptilian form and has abnormal features.

Avlyn's "normal" form, he looks like a normal anthropomorphic goat with some hidden features (will put a ref of later). He has hidden "fangs" that function like a snake's, folding back into the roof of his mouth when he doesn't need them. His hand hooves are usually soft and malleable, making them functional hands, but they can harden when on attack. His flesh is a neon cyan color, as he is a monster.

He is bigender, female-presenting dominant, and a lesbian.