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.
Forthwith that image vile of fraud appear'd,
His head and upper part expos'd on land,
But laid not on the shore his bestial train.
His face the semblance of a just man's wore,
So kind and gracious was its outward cheer;
The rest was serpent all: two shaggy claws
Reach'd to the armpits, and the back and breast,
And either side, were painted o'er with nodes
And orbits. Colours variegated more
Nor Turks nor Tartars e'er on cloth of state
With interchangeable embroidery wove,
Nor spread Arachne o'er her curious loom.
As ofttimes a light skiff, moor'd to the shore,
Stands part in water, part upon the land;
Or, as where dwells the greedy German boor,
The beaver settles watching for his prey;
So on the rim, that fenc'd the sand with rock,
Sat perch'd the fiend of evil. In the void
Glancing, his tail upturn'd its venomous fork,
With sting like scorpion's arm'd. Then thus my guide:
"Now need our way must turn few steps apart,
Far as to that ill beast, who couches there."