Russian icon design company TurboMilk offers free alien themed icon set for free download. They come for Windows, Macos and as PNG files. Icons can be freely used for personal noncommercial purposes. Give credits to http://www.turbomilk.com in case of public use
They have also two tutorials on icon drawing:
Other free icon sets by TurboMilk:
- Animals – 6 cutest icons ever
- Monsters – 7 mostroworld icons
- Underwater – Complete set of ocean deep icons