This one I can understand you being scared of. It was most likely that Fat Blue spider. I heard a lot of people on this forum were unnerved by the Fat Blue spider in that skit or the one on the Miss Muffet play. But this one had an Animal-type voice that made him more threatening
I can definitely understand this one. It's that eerie, dark, ominous music that plays during the first third of the skit, the setting of the Count's castle and the bats, etc.
It was most likely Dr. Rainbrain (or however you say it) and his insane Muppet Show-like behavior that made this one a bit scary for some people.