Vermin are creatures who operate on instinct, driven by simple needs such as food and reproduction. Most vermin attack only when hungry or threatened. Vermin are always neutral. Vermin are not governed by a human sense of morality. Vermin are mindless.

Vermin generally possess no treasure. For those that do, this treasure consists of possessions formerly owned by a creature that the monster has killed.^{MGWS 28} This type includes insects, arachnids, other arthropods, worms, and similar invertebrates.^{MG 199} Some animals like rats and mice are types of vermin.