"Famous" not necessarily meaning "D&D famous", it's been said that most monsters have D&D stats now, even if they didn't start in D&D. Mind you, if the D&D stats for a monster that originated outside D&D didn't really match up with the usual description, I wouldn't have a problem with those stats being abandoned.
I do like the idea that it might be Belkar adjacent, Bloodfeast perhaps, but I agree that the character is important as well as the species.Conversely, if the reveal is not dependent on recognizing the monster but instead on who the monster is as a character and why he chooses to do what he does, then there are suddenly no issues.
Famous in the real world, not in "Stick World", there have to be lots of monster types that haven't yet appeared in the comic.And all of this is ignoring that a "famous" monster doesn't make much sense given the circus scene, where his entire act is to just stand there, which somehow overwhelms virtually all viewers in various ways.