[Spike scratches his head, trying to figure out a way of describing an alicorn.]

Well, they're sort of a unicorn and a pegasus in one. They can use magic and fly. Alicorns are pretty rare. There aren't very many in Equestria. Most ponies are either a pony, or a unicorn, or a pegasus.

[Spike looks a little dubious about John's second comment]

I hope so...