In the Crisis Crossover starring Onslaught itself, he took three forms. The first was armor patterned on Magneto's with Spikes of Villainy added, completely encasing a body with the size and musculature of the Hulk. In the second form, the armor was the same, but adapted for a body that was now even larger, but more low-slung with much bigger forearms, and a bony face was visible, and balls of pure energy leaked out of his back and vents in the armor. His third form was disembodied pure energy, and he was defeated just pages after taking this form.
There was also an X-Men character whose power was transforming into whatever his opponent was afraid of (he may have even been named something like Nightmare). The twist was, he was a good guy, and actually otherwise reallynice.
That's Trauma from Avengers: The Initative who transforms into the fears of others. Nightmare is in fact his father.
There are many mutants in the Marvel Universe who can transform, some of them really transform into ugly, instead of cool or powerful ones. The morlock Scaleface for example was a normal woman who could transform into a giant lizard.
There was also Freakshow from the run of Excalibur set on Genosha. He was able to transform into various monstrous forms.
The Clown, a major enemy of Spawn, spends most of his time in the mortal realm disguised as a short, overweight circus reject. But piss him off, and he'll reveal his true form - the large, powerful demon known as the Violator - and deliver a No-Holds-Barred Beatdown upon your ass.
In Amulet, this happens to anyone who gets "devoured" by the amulets. So far we've seen Luger grow into a giant troll-like creature.
One of the EGM Hsu and Chan comics has the title duo getting roped into a wrestling match against Satan. He initially appears as a long-haired blonde in a white suit, but when the match begins, he reveals his true demonic form...a skinny, red-skinned guy with horns and pantaloons, not unlike Pitch.
The Sentry when he becomes the void, especially in Siege.
Batman villain, Scarecrow, is definitely one of these as Scarebeast.
X-Men villain Bastion has his own One Winged Angel form at the end of the recent Second Coming event. He doesn't last long once Hope Summers gets well and sufficiently pissed.
Jean Grey's Dark Phoenix may count as well. While wings are pretty standard for Phoenix nowadays, Dark Phoenix's fiery, psychic energy wings and her tendency to eat stars when she's hungry certainly qualify her for a number of things.
In Megalex, evil and beautiful Sapient Ship personality Shalise transforms into an absolutely terrifying monstrous version of herself. Her mouth fills with fangs, her ears become bat-like, she grows in size, and worst of all, her beautiful breasts become pendulous and rotten, tipped with corroded metal blades.