I'm a fan of the writings of Terry Pratchett and I love the perverse nature of Greebo, not to mention his rampant sexuality.
Greebo is Nanny Ogg's tomcat, he is feared in the forests of Lancre by all animals, and he is the father of most of the cat population in the area. Flea bitten, with scars all over his face and his partially present ears he is a huge swaggering bully of a cat - renowned for once chasing a bear up a tree. He has one green eye and one which is milky white, he is intelligent for a cat but uses three catagories to define the things he sees 'rape,fight or eat'. This is the cat that Nanny Ogg refers to as 'Mr. Puss-puss' (or Mr Fluffy) and sees in her head as a cute little bundle of fluff - yet everyone else calls him 'gerroffyerbugger!!'
In Witches Abroad he was magically transformed into a human, and appears as a swashbuckling, dastardly buccaneer with an eye patch who emanates enough sexual energy to power a television. He can speak individual words with a drawn out accent, and he was also pleased to find that his cat's agility had transferred over to his human form - as well as a set of retractable, razor sharp claws on each hand.
Following the conclusion of Witches Abroad, Greebo still changes into human form when under stress etc, usually Nanny Ogg would hear a scream, then the naked Greebo would run into the room and ask for milk and fish heads.
Before becoming human, Greebo once swallowed a vampire in the form of a bat.
"The bat squirmed under his claw. It seemed to Greebo's small cat brain that it was trying to change its shape, and he wasn't having any of that from a mouse with wings on."
Vampires are resilient. They may have managed to rise (repeatedly) from the grave, to survive being sealed in a glass bottle in vapour form and then thrown into the sea, to be powdered to dust and restored by the merest drop of b-vord... but if this vampire manages to rise from the cat, the normal action of biology and digestion means this only is the start of its troubles...
Apart from having been about 6 foot 3 inches (190cm) tall in my prime I resemble Greebo in no way, in fact I am quite soft hearted. I must admit to being a little jealous of his sexual appeal and energy though.
(Acknowledgements to http://wiki.lspace.org/wiki/Greebo and Terry Pratchett)