- Symbolically, Dante's rendering is a metaphor for fraud: a human face (honest and just) atop a monstrous personage
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.