You might be sorry to hear this, but that wasn't actually a clown (unless masterwork changes a LOT about the game). It was just a normal forgotten beast, because it was alone and in the caves. Clowns are much, MUCH worse than forgotten beasts.
Well, at first I suspected it was a "mere" forgotten beast (mind you, we're talking about cutebolds, a forgotten beast that would be relatively harmless to dwarves is horrifying to the little sentient vermin called kobolds. God's sakes they can't even make any decent armor or weapons and they're like a third of the size of dwarves) since, as you said, it was alone and in the caves. However, it is actually called a Panda Demon (as in there are potentially more than one of them, not just a unique name) and its tile is the tile used for demons, not forgotten beasts.
So perhaps masterwork changes something or I got a bug or what have you, but I'm 99% sure that the fell beast which devoured 6 kobolds in as many seconds is, in fact, a clown which has somehow escaped hell. I'm gonna go check it in legends right now actually.
EDIT: Yup, "In a time before time, Elemri began wandering the Underworld." "In the midwinter of 1, Elemri escaped from the Underworld." Seven or so more lines of "struck down the kobold with a bronze sling." That's in fact a demon that escaped from hell, settled in a random cave, and 100 years later a small tribe of kobolds blundered in and got brutally fuckmurdered. And I got to witness it. Hoo....rayy..... What are the odds that my first kobold camp play got me to witness what, as I understand it, is the rare coincidence of a demon escaping hell and NOT taking over some humie or gobbo civ, but rather, lying in wait for a hundred years just so he could shut down any hope I had of a peaceful cutebold coexistence with the world. Bastard.
So yeah. Can't sleep. Clown'll eat me.