This animal appears in the Monster Manual (2024).
Swarm. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Tiny rat. The swarm can’t regain Hit Points or gain Temporary Hit Points.
Bites. Melee Attack Roll: +2, reach 5 ft. Hit: 5 (2d4) Piercing damage, or 2 (1d4) Piercing damage if the swarm is Bloodied.