(not really a character but just some world building I guess)

Obsidian is based loosely on the puca, an irish nature spirit that often take the form of black horses or goats

Obsidian is one of the primordial nature spirits from the beginning of Equestria, one that quickly grew jealous of Celestia and Luna’s rule and irate at their claim to Equestria’s lands

Her true form is a small pixie esque feline but she found that this wasn’t nearly scary enough so she assumed the form a horse, much like Celestia and Luna

Celestia seals her away eventually to tartarus where she becomes the self proclaimed “keeper of the gates”