Question about Luigi's Mansion "Bowser"

Boo Destroyer

Koopa Troopa
Is it a puppet thing or a mechanism?

It was never actually classified in the game, but it appears no inner machinery was seen (King Boo driver seat, anyone?). So my guess is it would be the former.

Could be a Fake Bowser from SMB on the NES.
Hmm...good question.
Yeah, my guess is that it was a type of robot. Otherwise, it wouldn't have been able to screw its head back on after it was blown off.