"I was looking for Flesh-eatin' Slug Repellant - They're ruinin' the school cabbages."
-- Rubeus Hagrid (CS4)
Flesh-eating slugs are magical pests that, despite their name, seem to be vegetarian.
Hagrid once needed a repellent for Flesh-Eating Slugs that were getting into the Hogwarts cabbages, and went shopping for it in Knockturn Alley, as it was apparently unavailable in Diagon Alley (CS4).
They are apparently a frightening creature, since one student was scared enough of them to have their Boggart turn into one (PA7).
There really is a flesh-eating slug in the muggle world. It eats worms with its razor sharp teeth.
It's unclear why flesh-eating slugs would cause a problem for cabbages, unless they also eat plants. The Chamber of Secrets video game suggests that the spittle of the slugs is corrosive to human skin, so it may be that the slug itself does not eat flesh, but its secretions are dangerous.
From the Web
"Flesh-Eating Slug" on the Harry Potter Wiki