Betelgeuse himself...and not just when he's in his giant-snake form. In the movie he's Chaotic Evil, can change form at will into anything, and he's not above killing people.
Two words: Shrimp Hands.
Just the beginning of the second intro! The original one wasn't that bad, but the second opening starts out with Beetlejuice's rotting corpse sitting inside a coffin as he's summoned back to life by Lydia chanting his name three times. The look in his eyes as he awakens is disturbing.