In SLASH'EM, the peaceful Pegasus will appear when Medusa is killed in such a way that leaves a corpse; stoning her with her own reflection will not leave a corpse and thus not create Pegasus. Interestingly, while Pegasus is flagged as a unique monster, Medusa can be revived with turn undead and killed again, creating another Pegasus.
Pegasus can fly, has very high speed (the same as a warhorse), and has a decent level, making it an excellent mount. Pegasus is always created peaceful, making it simple to tame if one can cast charm monster.
Like a ki-rin, Pegasus does not eat and thus can go feral when mounted by any role other than a knight; unlike a ki-rin, he cannot wear amulets such as the amulet of reflection. A Yeoman can use the calm steed technique to increase Pegasus' tameness after each mount in SLASH'EM variants where this technique is functional.
Pegasus is a somewhst minor figure in the myth of Medusa and Perseus; he sprang from Medusa's blood after Perseus decapitated her in most versions, reflected by his generation method in SLASH'EM. In EvilHack, there is a crystal chest nearby containing the Bag of the Hesperides, further alluding to the myth.