[Greebo is] a huge, one-eyed tom who is covered with so much scar tissue that he looks like a fist with fur on it.  he can only be said to have ears because there’s no other word for the things left on top of his head.

He radiates genuine intelligence.  He also radiates a smell that could cause sinus trouble in a dead fox.

A magic spell has left Greebo with the ability to turn into human form.  He is then 6 feet tall, broad shouldered and leather clad, with rippling muscles.  he has long sideburns, a mane of black hair, a broken nose and a black patch over his bad eye.  A lot of the things which as a cat he would do quite naturally present him with all kinds of problems when he attempts them in human form.

–Stephen Briggs, “Terry Pratchett: HisWorld, A Brief Guide to Discworld”
(illustration by Paul Kidby)

(ACtually, he doesn’t gain clothes when he transforms, just his Nanny and her friends are good at making or getting him clothes that suit his Look.  Greebo just suddenly becoming A Naked Prowler is one of the problems he faces as a result of being afflicted with Spontaneous Transformation.)