Yes, this art is based on Undertale, but it's also based on something else.

Let me just say that if you KNEW where these monsters come from the moment you viewed this drawing, you probably deserve a Follow from me.

Key word: "KNEW". Yes, the text says they're "Outsiders", but try not to cheat and look up where they came from, then tell me you "knew" when you actually really didn't know in the first place. Thanks. :]