I think this video was brought up in the last thread about PETA, but it's on horse-drawn carriages in New York, not actual riding.
Watch it with sound, then watch it without. The script they give Lea Michele makes it sound like the worst possible thing you could ever do to a living creature. Making an animal bred for pulling things..actually pull things? How dare they. If you watch it without sound, it's a video of a horse doing its job completely content. Le sigh. Lea Michele Exposes Horse-Drawn Carriage Cruelty | PETA.org
My Psychology professor is completely against PETA because they've destroyed experiments he's been apart of, so we went on a rant about them in class one day. It ended with "If they have such a problem with doing medical experiments on animals, they can volunteer themselves. That'd be rid of them pretty fast." Bahaha.