It’s likely that Dante considers the monstrous beings in the Inferno to be those that have committed terrible sins and deserve to be punished. In particular, I think he considers those who have committed sins against God and nature to be monstrous.
The most notable example of a monster is Geryon, the guardian of hell’s third circle. Geryon is described as having three heads: one human and two animal heads (a dragon and a lion). Dante describes him as “the most cruel beast ever born.”
One more example of this is the Minotaur, who represents lust. For those who are unfamiliar with Greek mythology, the Minotaur was a monster with the head of a bull and the body of a man. He was kept in a maze on Crete, where he fed on human flesh until Theseus came along and killed him by placing his sword down his throat as he slept.
Dante also seems to consider sodomites monstrous because they commit an unnatural act: having sex with another man. Because this act goes against God’s design for humanity, Dante believes that these souls must be punished accordingly in hell.
Another example of Dante considering sinners monstrous is found in Canto IX when he encounters Ulysses’ men tied up under trees on an island after being turned into trees by Circe’s magic potion. Ulysses’ men were once human but became trees because they had no respect for their leader or for each other.
