Geryon, &, is one of the demon princes. He is one of three not to have his own lair, and therefore is not guaranteed to appear; he will only arrive via demon summoning or a monster casting summon nasties in Gehennom.
He will start out peaceful (unless you have Excalibur), and ask for payment.
Due to his low speed, most players should be able to kill him without taking damage.
Geryon is derived from a Dungeons and Dragons demon, which is in turn derived from an ancient Greek monster.
The ancient Greek sources about his appearance are inconsistent with regards to numbers of heads, bodies, and limbs. The D&D version has a snake-like body, hairy human hands, a human head, bat wings and a poisoned tail-tip.
Geryon is an arch-devil sometimes called the Wild Beast,
attacking with his claws and poison sting. His ranking in
Hell is rumored to be quite low.