they appear in speacial events (almost always as a special currency)
most of them can only be used in special events (on halloween, we collect spirits to trade for special halloween sets)

also, you have the abstracts, wich you can get at any time of the year by going to the purple tent in the village(between the beauty contest and the huge tree in the middle of the screen) and you can also use them at any time(as soon as you save enough of BOTH silver and gold abstracts) to buy a celestial pony egg or a pegasus pony egg or the special oracle sets.
you earn abstracts by clicking the withe pony and playing the games. you spend them by clicking the black pony and accessing the store.(i'd recomend you to save 30gold abstracts + 750silver to buy a female celestial egg)

so in short, they're a special in-game currency wich you can obtain more of by paying feez OR by playing the mini-games so you can get special trait sets.