Greebo is a character in Terry Pratchett's Discworld books. He is a foul-tempered one-eyed tomcat whose owner, Nanny Ogg, insists against all the evidence that he is a sweet, harmless kitten. In the course of the books, he has killed two vampires, eating at least one of them.

In Witches Abroad he was transformed, for reasons that seemed to be a good idea at the time, into a human. In this form he appeared as a tall young man with a beard, dressed in leather, and radiating sexuality. He was confused by the differences between human and cat morphology and social relationships, but picked some aspects up quickly.

In Maskerade he once more took human form, attending the Ankh-Morpork Opera in the guise of the Count Gribeau.

A Greebo (in Discworld) is also a young person who wears a leather jacket and rides a motorbike; an abbreviation of Grease Boy.

Modern day greebos can be found in large groups and can be spotted wearing, baggy jeans, mainly dark blue or black, "fat" shoes, some go to the extreme of wearing all black including jewelery and make up.

Pratchett gives this as the origin of the cat's name. His own definition of a greebo is "Someone who wants to be a Hell's Angel, but doesn't have the style."