And an interesting twist from Night Mare, book 6 in Piers Anthony's Xanth series: the protagonist, Mare Imbri, is a "Night Mare." When, in a climactic scene, she confronts a shape-shifting villain, he almost saves himself by shifting from human form into horse form. Imbri does eventually defeat him, but it becomes much, much harder to do so. It isn't just because she can't hurt a fellow horse either, it's because she's in heat at the time. (Sigh, Piers Anthony...)
Red Court vampires can either take the form of beautiful young men and women... or hideous black rubbery bat-creatures. The human form is an illusion, which is bad news for anyone who's infected.
Also, this is explicitly the main power of the Knights of the Blackened Denarii. They change from human into some horrible form for combat, with a twisted Angelic rune on their forehead and a second set of eyes. So far we have seen: A snake-man, a medusa-haired human/panther thing, a six-legged horned fanged bear, a normal human with a shadow that will strangle you, an obsidian statue, a feathery tentacled thing, a praying mantis with little praying mantises for blood, an emaciated grayish spiny humanoid, and various forms of big and ugly and scaly and hairy.
In a subversion, Nicodemus (the head Denarian) explicitly does not have a One-Winged Angel form- both because he's The Chessmaster by nature and would rather put his power into abilities other than raw strength, and because between the Artifact of Doom hangman's noose he wears (which makes him immune to conventional attacks) and his own skill as a warrior, even his base human form can overpowering anything if need be.
In James Swallow's Warhammer 40,000 novel Deus Sanguinius, Arkio starts to metamorphise under the influence of Chaos. Inquisitor Stele can contain it, but at the climax, fighting Rafen, he starts to bleed black blood, and then to change in form. Rafen shows him himself in a mirror and he breaks it in rage. Then the Spear of Telesto rejects him.
There's something of a sort like this in American Gods, as the deities seem to have a form that shows one of their believers whose faith was strong enough to make them real. Thus, Mr. Nancy/Anasi has this form of an African boy with a distented stomach and infected leg and Hinzelmann shows a child pierced by swords, which actually reflects his being created through human sacrifice. Interestingly, The Other Wiki novel for the character indicates that this was actually that character's true form in the original stories.
In the The Dark Hunters series by Sherrilyn Kenyon, the leader of the Dark-Hunters, Acheron, is in actuality an Atlantean god. This means that if he's pissed beyond all hope of redemption, he turns blue, gets black horns, nails, and lips, and rips the ever-loving everything out of most everyone. Bonus points for the ability to end the world should he go visit his mother in Atlantean Hell.
Visser Three used dozens of these over the course of the series, rarely using the same one twice. Fridge Horror ensues when you realize there are species of these for him to acquire DNA from.
Also, you realize just how awesomely Badass V3 is: you can only acquire DNA while in your native form, and while Andalites such as the one the Visser possesses have the slashing tail of doom, that'd barely inconvenience some of the nastier beasties the Visser has turned into - some of whom have projectile attacks like fire and venom and spears. A centaur with a pointy tail managed to get close enough to these things to actually touch. (Yeah, acquiring puts 'em into a trance... but you gotta touch 'em first.) One of them was actually a natural predator of his race, evolved to remove Yeerks from their hosts to eat.
In the finale of book 2, Fablehaven's artifact is guarded by a seemingly harmless cat. When you kill it, it is resurrected as a larger cat. This happens seven more times, until it has become a winged, three-headed, three-tailed monstrosity with snakes sprouting from its back that shoots acid.
When Gavin (a.k.a. the demon-dragon Navarog) scales up into his dragon form.