A sort of bellybutton was apparently found on a dinosaur named Psittacosaurus, one of the horned Ceratopsia family. The "umbilical scar" comes from the yolk sack attachment while inside the egg. I think I can see it in the Gorgosaurus illustration in the original AD&D monster Manual.
Thank you! I vaguely remembered reading many years ago that some oviparous species have these yolk sacks that can create bellybuttons, but the Wikipedia article only mentions mammals. Platypuses don't have bellybuttons.
But they're often there, because the trope of other races just being "Humans, but..." t
Painfully true in many cases, but it actually suits Githyanki who once were human according to official sources.
