As far as I know, most of Alice cooper's monsters were created especially for that episode (personally, I think that the electric mayhem should have been used as the back-up band). I know that two of those monsters were created before that episode, though:
Flower-Eating Monster: the dark purple monster. He was created for a sketch in a Julie Andrews special where he ate flowers, which is where his name comes from. He also appeared in the episode with Juliet Prowse, where he makes smoke come out of himself in At the Dance, and he was in the Vincent Price episode as well.
Chopped-Liver Monster: Okay, I don't know how to descripte him (check the character guide at
www.kermitage.com to see what he looks like), but he first appeared in a Pigs in Space sketch in the Cloris Leachman episode, which is where he got his name, and he later appeared in the episodes with Roy Clark (in a cameo appearance in that episodes Pigs in Space sketch), Raquel Welch (dancing with a dog, I think, in At the Dance), and Melissa manchester (in yet another Pigs in Space sketch, this time as a salesman from the planet Crab Nebula).
There was another member, a dark purple monster with a silver beak, who appeared in the episode with Anne Murray.