*SPOILERS* So if Twilight can simply transform beings... can't she just, you know, like, turn Spike into a pony? Would make a nice episode. Just saying.
Magical accident with Twilight trying to teach Sweetie some spells. Boom, Rarity is now a dragon. Spike, completely lovestruck, spends the rest of the episode teaching Rarity on what it's like to be a dragon, plus trying to convince her to stay one.