I've had a few goofy ideas and was looking to hear your other silly & joke monsters. Here's a couple of my favorite.

1) A Beholder that took over a Lich's lair. The Beholder refuses to acknowledge the Lich or admit that it's alive, as they already "Killed/beat them!"

Since the phylactery is still there, the Lich may reappear only to be completely ignored. The Beholder claimed the phylactery as one of its many trophies. It will completely admit it's a fully functioning magic item, but again will refuse that the Lich can use it.

If anyone acknowledges the Lich the beholder will get mad because they're acting weird, calling them "strange/mad/crazy." The Lich ignores the Beholder just because "it's games are beneath me" and will insult it if brought up. If the Lich insults the Beholder the Beholder will defend itself "NOT TRUE! I AM NOT! I BEAT YOU FAIR AND SQUARE!" Though if anyone acknowledges those responses, the Beholder becomes enraged while denying it, claiming it's being lied to and/or tricked.

2) My other silly one is a clan of Kobalts following a "Greater Dragon" or "Great Dragon." They will mention to characters that the "Poor Master" is strange/deformed/little bit off and not to mention it at all. After all the master is "Still Greater Than You!"

The so called "Poor Master" is actually a scaled Beholder that once claimed that it was "Greater Than Any Dragon!" and so the Kobalds mistakenly started worshiping it as a dragon.