Trying to find an unkillable scary clown monster...

[Dannyalcatraz]

I don't know, but now I'm giggling madly at the thought of a tarasque in greasepaint.

...emerging from a horse-drawn carriage...

 

[LiL KiNG]

Without reading through all the replies my first response to the OP's query was "Tim Curry from 'The Rocky Horror Picture Show' as a Lich...

That Taunting Haunt is pretty nasty though, I've been looking to run a horror/terror campaign for my group and that unpleasant critter might fit the bill nicely!