Here is a monster I'd forgotten about until I grabbed "The Vanishing Tower" by Michael Moorcock as a quick in-between-non-fiction book read - the Oonai.
"Creatures of Chaos. In Melnibone they are called the Oonai. They can change shape at will. A sorcerer of great mental discipline, of superlative powers, who knows the apposite spells can master them and determine their appearance."
These creatures shapeshift into the form of - and gain the powers of - things up to and including Melnibonean dragons with their napalm-like venom. They attack individually even though they appear in groups, and can only be banished by a Lord of Chaos.
But, when slain - which they can be with normal swords - they turn back into "heavy, black, pig-like creature[s]" with ugly, lumpen bodies.
I'm a big fan of shapeshifters, but it's amusing they're just ugly lumpy black pigs when they're not anything else. It beats the usual "formless mass" that I've seen before (and used myself.)
Now just to stat them up secretly and use them in DF . . .