The gameplay of Diablo II is built on the backs of thousands of slaughter monsters, and if these guys ween't fun to look at, fight with, and massacre, the game wouldn't be any fun. Fortunately, the various demons, beasties, and undead enemies in Diablo II are quite enjoyable, and profitable, to work over.
See the Monsters Category for links to the individual pages of every monster in the game.
For fun, check out the pages in our monsters archive, with pre-release info and screenshots from 1997-1999. It's curious to look back and see how much the monsters were moved around during game balancing. Razorbacks in Act One? Fallen and Giant spiders in Act Two? No-delay on Dark Lords casting Meteors?