Childhood was fraught with surprise visits from the devil. He must have a really good agent.
Hades - "Hercules"
More "sassy gay friend" than evil villain, Disney scrubbed most of the scary right of the god of the underworld.
The Red Guy - "Cow and Chicken"
A scam-artist shapeshifter who walks on his butt? Disturbing, but not because he's scary.
Aku - "Samurai Jack"
An immortal dark god with superhuman strength, the power of interstellar travel, laser-beam eyes, and control of the elements who dabbles in necromancy? That is how you do scary shapeshifting.
Katz - "Courage The Cowardly Dog"
There's just something about his calm cruelty that lingers with you well after the show is over.