You could give ambiguous riddles, and hint at the trick by having the creature get flustered and angry when someone provides an answer that is correct but not the expected answer.

Like "I am a key that opens no door."

There's monkey, donkey, turkey, piano key, musical key, whiskey, malarkey, lackey, jockey, computer key, keystone, typewriter key, and probably some more that they will think of.

There are a bunch of riddles that can be made more ambiguous by leaving out one of the lines. Getting the riddle wrong could also be a hint that the creature is not as clever as he thinks.

There's also the riddle my grandfather like to tell.

What is red, you hang it on a clothesline, and it has four legs?

A fish!

But a fish isn't red! (Well, you could paint it red and then it would be)

You don't hang fish on clotheslines (It's mine, I can do what I want with it)

Fish don't have four legs!! (Yeah, I threw that in there because I didn't want the answer to be too obvious.)

Make sure the players understand that it's not just you who is the idiot.