Just to add to the swarmy goodness, here's a critter from my game:
Thorn Squirrel Swarm
Tiny Animal (Swarm)
Hit Dice 4d8 (13hp)
Initiative +6
Speed 50 ft. (10 squares)
Armor Class 16 (+2 size, +2 dex, +2 natural), touch 14, flat-footed 14
Base Attack/Grapple +3/-
Attack Swarm (1d6 + Poison and Wounding)
Full Attack Swarm (1d6 + Poison and Wounding)
Space/Reach 10 ft, 0 ft.
Saves: Fort +4 Ref +6 Will +2
Abilities: Str 2, Dex 15, Con 10, Int 2, Wis 12, Cha 2
Skills: Balance +10, Climb +10, Hide +14, Listen +6, Spot +7
Environment: Temperate Grasslands
Organization: Solitary, pack (2-4 swarms) or infestation (7-12 swarms)
Challenge Rating: 3
Treasure: None
Alignment: Always Neutral
Advancement: None
Level Adjustment: --
Languages: None
Special Attacks: Distraction, wounding, poison
Special Qualities: Half damage from slashing and piercing, low-light vision, scent, swarm traits
Feats: Alertness, Weapon Finesse
Description
Thorn squirrels are native to the grassy plains and uplands of Graesholm. Most of the time, they are innocuous rodents with a lifestyle similar to the prairie dog. However, they occasionally experience population explosions. When this happens, or any time they are forced to live in very crowded conditions, they transform into a menace. Their skin develops hardened nodules which grow into wicked, horn-like thorns. They become very aggressive and develop a taste for meat. Eventually swarms of the creatures scatter across the plain, moving at lightning speed and bringing down anything in their path. If no food is available they will eventually turn on each other. The grasslands are afflicted with thorn squirrels every 10-12 years on average. An infestation usually lasts about two weeks.
Combat
A thorn squirrel swarm seeks to surround and attack any prey it encounters. Thorn squirrels will eat grain, fruit or carrion, but prefer living prey. A swarm deals 1d6 points of damage to any creature whose space it occupies at the end of its move.
Distraction (Ex): Any living creature that begins its turn with a thorn squirrel swarm in its square must succeed on a DC 12 Fortitude save or be nauseated for 1 round. The save DC is Constitution-based.
Poison (Ex): A thorn squirrel’s thorns exude a natural toxin (to which the squirrels themselves are immune). Any creature injured by a thorn squirrel swarm must make a DC 12 Fortitude save or be paralyzed for 1 minute. A creature who successfully saves is immune to that swarm’s poison for 24 hours.
Wounding (Ex): Any living creature damaged by a thorn squirrel swarm continues to bleed, losing 1 hit point per round thereafter. Multiple wounds do not result in cumulative bleeding loss. The bleeding can be stopped by a DC 10 Heal check or the application of a cure spell or some other healing magic.