Perseus was the hero of Greek legend who killed Medusa. He appears in NetHack in statue form on the down stair of Medusa's lair. Either he failed to kill her this time, or Medusa keeps a statue of him to remind her of past failures (if his statue is carrying a shield of reflection, it's hard to say how he went wrong...).
In the first variation of Medusa's lair, there is a 75% chance of his statue containing a cursed +0 shield of reflection, a 25% chance of cursed +0 levitation boots, a 50% chance of a blessed +2 scimitar, and a 50% chance of a sack.
In the second variation of the level, there is a 25% chance of his statue containing a cursed +0 shield of reflection, a 75% chance of cursed +0 levitation boots, a 50% chance of a blessed +2 scimitar, and a 50% chance of a sack.
All these chances are independent of each other.
Perseus is not a monster. If you cast stone to flesh on his statue, you create "a knight named Perseus", not a unique monster called Perseus.