There's only one thing creepier than full-body Muppets...
6) The Mutations
And that's full-body Muppets that are way into choreography and reminiscent of the horrible mutants from Beneath the Planet of the Apes.
The song above would have you believe that Muppet monster Frazzle "looks ferocious but he's really fine." That may be true, but with his sinister eyes and razor sharp teeth I'm too busy trying to maintain control of my bladder to find out what he's really like.
4) Uncle Deadly
Uncle Deadly spends most of his appearances on The Muppet Show haunting the Muppet Theater. As such, he is a shoo-in to grace the top five of this list. Scary looks are one thing, but when you pair them with Uncle Deadly's "ac-tor" persona you've got the making of a truly nightmarish character.
Yorick is a one-and-done Muppet whose sole appearance was in the episode of The Muppet Show that was hosted by Christopher Reeve. Jump to 3:58 in the above clip to witness how Reeve's attempts to bring Hamlet to life are thwarted by the Muppets, including Shakespeare's famous skull, rendered here as an awesome (from my 2013 perspective) and horrific (circa 1980 me) puppet.
2) Swedish Chef
At one point becoming popular enough to spawn his own breakfast cereal, the Swedish Chef is arguably the silliest of all the Muppets. (Certainly one of the most popular). Yet beyond all of the goofy faux-Swedish lies a strange element to the otherwise puppety character: his hands are those of a human. As a child, I became so fixated on this aspect of the Chef that I couldn't focus on what dish he was trying to make whenever he appeared on The Muppet Show. Those hands...why? Was there a person trapped under his skin, trying to somehow cook his way to freedom? Or was I just an over-sensitive kid whose unblinking focus on those hands caused uneasiness? Such are the bizarre worries of adolescence. Those things still do kinda creep me out though.
1) Gary Cahuenga
Have we learned nothing from Magic? You know why Muppets Tonight character/living ventriloquist dummy Gary Cahuenga didn't appear in The Muppets? Because he's serving 25 to life for choking Robin to death.