Greebo is a fictional character in Terry Pratchett's Discworld books. He is first introduced in Wyrd Sisters. He is a foul-tempered one-eyed grey tomcat whose human, 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 the novel Witches Abroad:
And in Maskerade, Magrat recalls when Greebo once killed an elk.
In Witches Abroad he was transformed, for reasons that seemed to be a good idea at the time, into a human. He was confused by the differences between human and cat morphology and social relationships, but picked some aspects up quickly. In this incarnation, the character is reminiscent of Red Dwarf's Cat; he looks like a 'beautiful, brainless bully' who has raided a leather goods store for the discerning pirate, and appears ready to unbuckle any amount of swash, radiating an aura of raw sex that can be felt several rooms away, and of a Megawatt magnitude. He is typically covered in scars, has retractable nails (or more accurately, claws) and maintains a slitted left eye.
The incident left him with an unstable phenotype, that is, a tendency to temporarily switch forms when completely cornered, which causes some excitement. In Maskerade he once more took longer-term human shape, attending the Ankh-Morpork Opera in the guise of the Le Comte de Gribeau.
In a passing reference in Witches Abroad Nanny Ogg said that Greebo was a demon ("Just between you and me, he's a fiend from hell"), possibly indicating that Greebo is indeed demonic, partially due to Nanny's insistence otherwise that Greebo is a gentle kitten. In addition to his other qualities, he is pursuing a study in inbreeding with all of the other cats in Lancre that are related to him (which by dint of some effort is all of them, as he is currently every male ancestor for at least 30 generation), which may be involved in the "raw sex" aura he possesses mentioned above. He has also seriously surprised a she-wolf (also killed and eaten half of one before) and chased a female bear up a tree.
To date, Greebo is known to fear only three things:
In Terry Pratchett's Johnny Maxwell series, Mrs. Tachyon owns a cat named Guilty who has many qualities similar to Greebo.