Well, horses are more specifically usually symbols of repressed sexuality, probably because of the fact that you're trying to control them, sometimes successfully, sometimes not. And let's not forget their obvious musculature and power. Not to mention that they fit between your legs. In mythology it gets even worse, when unicorns carry a giant phallus on their foreheads and can only be ridden by virgins. Pegasus isn't quite as good, though, although flying is often seen as a symbol of sexual desire.

And I'm guessing that My Little Pony is just an experiment in marketing, just like every other kid product made past 1985.