Particles! I used the prefab of the clouds and played around with some of the parameters till I got something interesting going on. The bat is kind of in creepy eye-only mode right now, but there he is. I brought down the quality of the resolution for the sake of my computer's power. This is super short. I exported 48 frames (which took FOREVER) and then set it to play at 12 fps instead of 24. So, here is four seconds of clouds. Enjoy!

Bat Clouds from Lady Subrosa on Vimeo.