(In no specific order - thanks to Topher's Cereal Character Guide for pics and info)
The Lucky Charms Leprechaun
Not because he looked any freakier than any other leprechaun, but more the pervy way he was after MY Lucky Charms all the time.
Frankenberry
Okay, all the Monster Cereals by General Mills were infinitely cool and hold a nostalgic place in my memory. I even set up a Facebook page in their honor. And while they were all creepy, Frankenberry was the most freaky. Frankenstein's monster had nothing on the pink, doughy-looking, metamorphosis that was Frankenberry.
Sir Grapefellow and Baron Von Redberry
Let's celebrate WWI flying aces with crazy fruit cereal. If I don't like the cereal, will they be strafing my front lawn?
Crazy Cow
This bi-polar buck-toothed freakshow would turn your milk chocolate or strawberry depending on which side it/they were facing - Sybil anyone?
Colonel Corn Burst and Hattie the Alligator
What demented mind put this pair together? You've got a crudely-drawn alligator that's about to turn its head and swallow a microscopic adventurer. Either of these characters would be freaky enough in their own right. Together they're a total "corny" burst.
Cheeri O'Leary and Joe Idea
A demented beauty pageant tart and a boy who looks like he's got a John Merrick thing going on... yeah, that'll sell me Cheerios!
Mr. Wonderfull
From Mr. Wonderfull's Surprize Cereal, not only did he preach the doctrine of incorrect spelling, but here's a perv that needs be put into a registry before he moves into your neighborhood.
Magic Hat
Cool beer. Psychotropic-induced cereal character. Don't take the brown acid folks.