Okay, he thought as he opened the book. If I've got this right, there are four types of pony. The plain ones, the ones that look kind of like horses back home, are called earth ponies. The ones with horns are unicorns, the ones with wings are pegasuses, and the ones with both horns and wings are princesses. They're all born that way, and they all live together. He frowned and picked up his coffee, trying to think back to high school biology. I got the impression that there's a bit of unpredictability when it comes to offspring: if the first picture is accurate, that's their version of a nuclear family, and it has three of the four types of pony. The princess type is probably rare, and has higher social standing, which is why everyone was deferential towards Luna last night.