What do you think are the "crystal ponies" ?

I agree 100%.

Your answer is really good. Though what I was thinking is maybe the crystal ponies are a sub race of unicorns. I remember in A Canterlot Wedding the evil Princess Cadance put Twilight in a crystal cave where she (evil Princess Cadance) said and I quote "The caves beneath Canterlot, once home to greedy unicorns who wanted to claim the gems that could be found inside. And now, your prison." So I think maybe the crystal ponies are these unicorns and maybe they were banished from Canterlot because of there greedy nature or they were given a home in which they can foster their love of crystals/gems. So this is my idea.

Crystal ponies, eh? Perhaps they're a heavenly kind of pony created by the goddess (Ms. Faust), who came to share their happiness and culture with the other ponies down on the ground. Shortly afterwards, they then ascend back into the heavens.

IDONTKNOWSOILLMAKESOMETHINGUP!
I'll go with this: "Crystal Ponies" are drug-addicted ponies, y'know cause "crystal", like the meth, and "ponies" because, well, yeah... So anyway, these "Crystal Ponies" were exiled because Celestia deemed them to be a bad influence to the other ponies. But, out of pity, there is a festival held in their honor, during which they can reunite with their family members who did not sucumb to the drugs...