It's an innate supernatural ability, mostly added to enable them to use Gust of Wind without killing themselves. Gust of Wind, of course, is there for the butterflies and hurricanes thing. Thank you for bringing this up, I'll edit something in.
Interestingly, Disguise self doesn't specify that. It specifies that you can make yourself appear different, but can't change your body type. I'd assumed that the swarm could simply arrange itself into, say, a roughly humanoid shape and then use Disguise Self to look like an Elf. Although whether simply shaping itself into a humanoid shape (or some other shape, Disguise Self doesn't actually limit itself to creatures either) would be enough to allow this might be up to a DM's interpretation. Maybe I should add some sort of Ex ability specifically enabling this? Or remove the Spell-Like and just give it a Su ability to do the whole thing? What do you think?
EDIT Added in an Ex ability to explicitly allow it to use Disguise Self like that. I'm not sure about the wording, so if anyone has any suggestions...
Also, do you think the save DCs, particularly for Bewilderment, are a bit high for 4th level characters?
EDIT2 Added the slightly nastier version in a spoiler box underneath the main entry. It's a bit more durable and has a lot of nasty weather effects. I'm still pretty unsure about the numbers on this one, so critique would be nice. The really nasty version is still being worked on.
I also have a slew of other Chaotic Outsider ideas drifting around, but I'm not sure if enough folks are going to want to see them to make it worth stating them up properly. Dancing Stars, Formless Things and Entropic Maws, to be exact. Is anyone interested in seeing them?