Pegasus is a monster added in SLASH'EM. It 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, Pegasus is not actually unique, as Medusa can be revived with turn undead and killed again, creating another Pegasus.
As a steed
Pegasus can fly, has very high speed (the same as a warhorse), and has a decent level, making it an excellent mount. Like a Ki-rin, Pegasus does not eat and thus can go feral when mounted by any role other than a knight. A Yeoman can use the calm steed technique to increase Pegasus' tameness after each mount (if you are playing a SLASH'EM variant where this technique is functional.) Pegasus is always created peaceful, making it simple to tame if one can cast charm monster.
Pegasus will always leave a corpse, meaning that provided he isn't disintegrated or killed over lava, one can always revive him. Unfortunately, he will not wear an amulet of reflection, unlike a ki-rin.