This is my first attempt at playing around with swarms. It's essentially a modified rat swarm made with a statistics block for a mouse I found in my archive.
MOUSE SWARM
Fine Animal (Swarm)
Hit Dice: 4d8 (13 hp)
Initiative: +2
Speed: 10 ft. (3 squares), climb 10 ft.
Armor Class: 20 (+8 size, +2 Dex), touch 14, flat-footed 12
Base Attack/Grapple: +3/-
Attack: Swarm (1d6 plus disease)
Full Attack: Swarm (1d6 plus disease)
Space/Reach: 10 ft./0 ft.
Special Attacks: Distraction
Special Qualities: Immune to weapon damage, low-light vision, scent, swarm traits
Saves: Fort +2, Ref +4, Will +1
Abilities: Str 2, Dex 15, Con 10, Int 2, Wis 12, Cha 2
Skills: Balance +10, Climb +12, Hide +24, Listen +6, Move Silently +10, Spot +2
Feats: Alertness, Weapon Finesse
Environment: Any Land or Underground
Organization: Solitary, pack (2-4 swarms), or infestation (7-12 swarms)
Challenge Rating: 2
Treasure: None
Alignment: Always neutral
Advancement: None
Level Adjustment: -
You hear a loud squeaking sound. Thousands of minute gray rodents scuttle around in different direction, confusion and fear in their beady eyes, and their diminutive teeth chattering away.
Combat
A mouse swarm seeks to surround and attack any warm-blooded prey it encounters. 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 swarm in its square must succeed on a DC 12 Fortitude save or be nauseated for 1 round. The save DC is Constitution-based.
Skills: A mouse swarm has a +4 racial bonus on Hide and Move Silently checks, and a +8 racial bonus on Balance, Climb, and Swim checks. A mouse swarm can always choose to take 10 on all Climb checks, even if rushed or threatened. A mouse swarm uses its Dexterity modifier instead of its Strength modifier for Climb and Swim checks.
Other:
A Mouse Swarm can be summoned with Summon Swarm
