Monsters (Diablo I)
There are over 20 different kinds of monsters in Diablo I. Those in the original Diablo each come in four different subspecies specific to each breed, while those in Hellfire only come in one subspecies per breed.
Most monsters in the original game also have a unique version of their monster type, while some have more than one and a few have none. In addition to that, The Butcher, The Skeleton King and Diablo are technically speaking normal monsters, but only come in unique varieties (similar to how bosses work in Diablo II).