Part-troll

Apparently, in the wizarding world, humans are able to cross-breed with trolls. The fact appears to be canon, however how, or why this occurs is not known.

Ronald Weasley mentioned that one of the snatchers was part-troll in Harry Potter and the Deathly Hallows.

Behind The Scenes
The term 'part-troll' could just be a derogatory phrase used to insult wizards and witches.