The beast that carries Dante and Virgil to the eighth circle is Geryon. In Canto XVII of Inferno, Geryon rises from the abyss after Virgil calls him, then bears both poets down the great cliff into Malebolge, the eighth circle of Hell.
Dante gives Geryon a frightening hybrid form: a human face, hairy arms, a reptilian body covered in patterns, and a scorpion-like tail. The apparently honest face is crucial to the symbolism. Geryon represents fraud, whose outward appearance may seem trustworthy while concealing a poisonous intention.
Dante adapted the creature from classical mythology, where Geryon was a triple-bodied giant who owned cattle in the far west and was killed by Hercules during one of the hero's labours. Dante transforms that ancient giant into a flying monster associated with deception.
A common mix-up is Minos, who judges and assigns souls to Hell's circles, and Charon, who ferries souls across the Acheron. Neither transports Dante and Virgil to the eighth circle: that descent is Geryon's distinctive role.