How we cite our quotes:
He threw the sinner down, then wheeled along
the stony cliff: no mastiff’s ever been
unleashed with so much haste to chase a thief. (Inf. XXI, 43-45)
Already inhuman by nature, this demon confirms bestiality when his savagery towards the struggling sinner spurs Dante to compare him to a fierce guard dog, a mastiff. His "unleashed" behavior suggests that, like the incontinent sinners, the demon is unable to contain his baser instincts.
"Mule that I was, the bestial life pleased me
and not the human; I am Vanni Fucci,
beast; and the den that suited me – Pistoia." (Inf. XXIV, 124-126)
The thief Vanni Fucci admits to his subhuman existence by describing himself as a "beast" living in a horrid "den" of a city. Although he has much to be ashamed of ("mule" here, Mandelbaum suggests, means "bastard" or "illegitimate child," as Fucci was), Vanni Fucci later lashes out at Dante in wrath, cursing him with an ominous prophecy simply out of spite. It seems that he takes a measure of perverted pride in his actions.
As I kept my eyes fixed upon those sinners,
a serpent with six feet springs out against
one of the three, and clutches him completely.
It gripped his belly with its middle feet,
and with its forefeet grappled his two arms;
and then it sank its teeth in both his cheeks;
it stretched its rear feet out along his thighs
and ran its tail along between the two,
then straightened it again behind his loins.
No ivy ever gripped a tree so fast
as when that horrifying monster clasped
and intertwined the other’s limbs with its.
Then just as if their substance were warm wax,
they stuck together and they mixed their colors,
so neither seemed what he had been before; (Inf. XXV, 49-63)
The thieves’ punishment of being transformed into hideous beasts (like "a serpent with six feet") reflects their lack of respect for boundaries. Because in life, they did not recognize other people’s property as off limits, they are punished with a violation of their own physical boundaries; they must become one another, sticking together and "mix[ing] their colors" like "warm wax." Dante considers the ability to distinguish between one’s property and another’s a purely human one, not recognized by beasts; thus, by failing to respect boundaries, the thieves forfeit their humanity and become beasts, both physically and spiritually.