For all the fantastic creatures contained in Hagrid’s textbooks, there are actually very few non-human characters in Harry Potter. Humans (wizard or muggle) remain the main heroes and villains of the story, and many monsters that might inhabit other series remain at the periphery. The Inferi seem to stand in for zombies and vampires and other undead—but then we never see much of them. Lupin remains our lovable werewolf—but outside of book three, there isn’t much to be made of this. Mermaids remain at the bottom of the lake; dragons and hippogriffs seem to be constantly getting shipped to other parts of the world in order to avoid liquidation.
So the most interesting additions to the Potterverse are the House-Elves—distant seconds are the Centaurs and the Goblins. The House-Elves create an ever-progressing revision in the series. In book two, we’re first introduced to Dobby, clever and good-natured, if bumbling; in book five, we’re introduced to Kreacher, a servant as befouled and bigoted as Dobby is sweet-hearted and loyal. They both serve important plot points in book seven where—despite having astounding magic—the Dark Lord seems to ignore them almost entirely.