Unicorn

Unicorns are pointy horses. They don't have one corn though, for some reason, they have one horn. They are magical creatures. The only one we've come across was a refugee from the Land of the Faeries, the Queen's pet. It stank atrociously, and was maddened on reaching the "real" dimension of the Disc to find that time - to which it was not ordinarily subject - was passing and that the sky was only blue...and the Queen's unthinking cruelty was no longer in sole command of its head. In short, it was insane - as evidenced by its charging Granny Weatherwax - a thing no mortal creature would ever do. Animals are extremely good at knowing what would be more-than-suicidal. The unicorn was taken by Granny Weatherwax to Jason Ogg's Smithy, where she had him shoe it with silver horseshoes.

Annotation

Whilst mythical, there's an enormous catalogue of references to unicorns on Roundworld.

And there's one thing that everybody knows about unicorns: it takes a virgin to be able to tame them, hence the ease with which Granny does so, leading it to Jason Ogg's smithy tethered by just one of her silver hairs. A unicorn hunter was among the heroes who visited Ankh-Morpork during the attack of the dragon, but decided that the reward offered was not high enough. He was described as a quite man with a shy smile.