You can't have butterfiles, they'd be vermin typed, not animal
That aside, I'd just extrapolate it from locust swarm, making it weaker :
BUTTERFLY SWARM
Diminutive Vermin (Swarm)
Hit Dice: 2d8–2 (7 hp)
Initiative: +4
Speed: 10 ft. (2 squares), fly 30 ft. (poor)
Armor Class: 18 (+4 size, +4 Dex), touch 18, flat-footed 14
Base Attack/Grapple: +4/—
Attack: -
Full Attack: -
Space/Reach: 10 ft./0 ft.
Special Attacks: Distraction, Harmless
Special Qualities: Darkvision 60 ft., immune to weapon damage, swarm traits, vermin traits
Saves: Fort +2, Ref +4, Will +0
Abilities: Str 1, Dex 19, Con 8, Int —, Wis 10, Cha 2
Skills: Listen +2, Spot +3
Feats: —
Combat
A butterfly swarm surrounds and distracts any living prey it encounters.
Harmless (Ex): A butterfly swarm is physically unable to attack, making it harmless.
Distraction (Ex): Any living creature that begins its turn with a butterfly swarm in its space must succeed on a DC 10 Fortitude save or be nauseated for 1 round. The save DC is Constitution-based.
Skills: A butterfly swarm has a +4 racial bonus on Listen and Spot checks.
PS. I made up the Harmless property, it doesn't seem right being attacked by butterflies... what are they going to do, kamikaze you ?! Anyway the above would be fine (and quite possibly underpowered) for a first level druid to take, DM allowing.
PPS. Oh , the distraction ability is DC 10 + 1/2 racial HD + con mod (so 10 + 1 - 1 in this case). So as it gains HD, remember to increase the DC by 1 for every 2 HD it gains. Likewise if you somehow justified a killer butterfly swarm and got rid of the harmless quality I gave it, it'd be doing Swarm Damage (1d6) as it is, then 2d6 at 6HD, 3d6 at 11HD, etc