Swarm

Source Bestiary pg. 347
A swarm is a mass or cloud of creatures that functions as one monster. Its size entry gives the size of the entire mass, though for most swarms the individual creatures that make up that mass are Tiny. A swarm can occupy the same space as other creatures, and must do so in order to use its damaging action. A swarm typically has weakness to effects that deal damage over an area (like area spells and splash weapons). Swarms are immune to the grappled, prone, and restrained conditions.

Monsters

Algea, Amoeba Swarm, Animated Silverware Swarm, Army Ant Swarm, Bone Skipper Swarm, Bore Worm Swarm, Brood Leech Swarm, Centipede Swarm, Clacking Skull Swarm, Cobbleswarm, Cockroach Swarm, Consonite Choir, Fen Mosquito Swarm, Feral Skull Swarm, Fire Jellyfish Swarm, Flea Swarm, Gibtas Spawn Swarm, Hellwasp Swarm, Hermit Crab Swarm, Monkey Swarm, Penqual, Piranah Swarm, Rat Snake Swarm, Rat Swarm, Raven Swarm, Scorpion Swarm, Silverfish Swarm, Sorcerous Skull Swarm, Spider Swarm, Sportlebore Swarm, Squirrel Swarm, Sturzstromer, Tick Swarm, Tooth Fairy Swarm, Trilobite Swarm, Vampire Bat Swarm, Viper Swarm, Wasp Swarm, Wizard Sponge, Worm That Walks Cultist