swarm

A swarm is composed of multiple creatures but functions as a single creature. A swarm can occupy the same space as another creature, and an enemy can enter a swarm’s space, which is difficult terrain. A swarm cannot be pulled, pushed, or slid by melee or ranged attacks.
    A swarm can squeeze through any opening large enough for even one of its constituent creatures. For example, a swarm of bats can squeeze through an opening large enough for one of the bats to squeeze through.

Published in Monster Manual 2, page(s) 218, Monster Manual, page(s) 283, Rules Compendium, page(s) 316, Monster Vault: Threats to the Nentir Vale, page(s) 125.