okay, now if you give me your bank account details....=P
jokes, its simply a random thing. its some remnant of a joke left over from the very early days of the game. you explore planets and then you find them, there isnt even a popup or anything telling you. when you mouse over a planet you will see a small icon of a pink pony that says Space Ponies! then you get the acheivement.
it doesnt actually do anything for you though...
i actually got so pissed off that after a year and a half of games i never found it that i went into the acheivements file and manually altered it so the i got the Space Ponies Achievement
p.s. thanks for the Karma =D