In Greek mythology, Geryon was a three-headed (or three-bodied) giant Hercules slew during one of his labors. Dante places him in the seventh circle of hell and presents him as a manifestation of fraud (Canto XVII).
I find this confusing because the Greek myths regarding the giant do not show him to be deceptive or fraudulent. He's just unlucky Hercules was asked to steal his cattle. Why did Dante decide to use Geryon as a symbol of fraud?
Translator-poet John Ciardi (Dante, The Inferno, Signet Classics, 2001, p. 139) offers the following annotation:
GERYON. A mythical king of Spain represented as a giant with three heads and three bodies. He was killed by Hercules, who coveted the king’s cattle. A later tradition represents him as killing and robbing strangers whom he lured into his realm. It is probably on this account that Dante chose him as the prototype of fraud, though in a radically altered bodily form. Some of the details of Dante’s Geryon may be drawn from Revelations, ix, 9–20, but most of them are almost certainly his own invention: a monster with the general shape of a dragon but with the tail of a scorpion, hairy arms, a gaudily-marked reptilian body, and the face of a just and honest man. The careful reader will note that the gaudily-spotted body suggests the Leopard; the hairy paws, the Lion; and that the human face represents the essentially human nature of Fraud, which thus embodies corruption of the Appetite, of the Will, and of the Intellect.
Multiple heads or bodies in the Classical depiction can indicate deception strategies. (For instance, one head could befriend you while the others plot your destruction. If multiple bodies, one could attack from behind.)
For example, Ravanna, the rakshasha king in the Ramayana, is famously many-headed and employs deception in capturing Sita to lure Rama to Lanka.
There is a fragment from Stesichorus that implies Heracles himself used stealth to defeat Geryon, which seems strange per Heracles' legendary strenght:
"In his mind he distinguished [Herakles who was deliberating on whether to kill Geryon by stealth or in an open fight,] . . ((lacuna)); it seemed to him to be much better . . ((lacuna)) to fight by stealth . . ((lacuna)) against the mighty man; . . ((lacuna)) (crouching) on one side he devised for him . . bitter destruction;
Source: Stesichorus, Geryoneis Fragment S15 | Theoi
Possibly there was something more to Geryon than his physical might, prompted Heracles to be cautious and employ an indirect method of attack.
The impetus for Dante's reconfiguration may have been toward a more compact and visceral form for the metaphor. "Monster with a human face" seems to be critical component.
