I don't know what the big deal is about with these Space Ponies, to be honest. They don't actually do anything.
They are found the same way as any other planetary bonus, through exploration. Their little icon is a pink or white pony (don't know which, I'm colour blind) that has the classic 'fishbowl' helmet on.
The description just says: Space Ponies!!
Found them before I even knew there were achievements. Meh.
Just keep looking guys. It's random.