Inferi revealed to not be zombies by author J.K. Rowling in Day 10 of 12 Days of Pottermore.
We go into the cave in which Harry and Headmaster Dumbledore are confronted with zombie beings called Inferi. In this section of Harry Potter and the Half-Blood Prince, they are in search of one of Voldemort’s Horcruxes, but come across some reanimated corpses.
“The Inferi whome Harry and Dumbledore encounter in the depth of the lake in Harry Potter and the Half-Blood Prince were, when alive, mostly vagrant, homeless Muggles whom Voldemort had murdered for the purpose during his first rise to power, although some were the earthly remains of wizards or witches who ‘disappeared’ without explanation.”
On JKR’s reasons for not calling them zombies:
They are not part of British folklore.
“While zombies of the Vodou tradition can be nothing more than reanimated corpses, a separate but related tradition has it that the sorcerer uses their souls, or part of their souls, to sustain himself. This conflicted with my Horcrux story, and I did not wish to suggest that Voldemort had any more use for his Inferi than as guards of his Horcrux.”
She also had a somewhat humorous reason for not wanting to use the word zombie, and it was in regards to pop legend Michael Jackson.
“I’m part of the ‘Thriller’ generation; to me, a zombie will always mean Michael jackson in a bright red bomber jacket.”
Thus, like many other words in the book, Rowling derived the name from Latin:
“The name Inferius was a play on ‘Inferus’, which is Latin for ‘below’, but with an obvious connotation of being ‘lesser’ than a living human.”
In the Inferi section of Pottermore, fans can collect seaweed in the center of the water. There’s also a pebble worth collecting next to the foot of the Inferi closest to Dumbledore’s left arm!