I've seen plenty of horses in swimming pools. I'd love to do it. Swimming is great for all kinds of critters. I've always seen a ramp into water, a swimming lane, and then a ramp out. Have you seen a video of a horse on a treadmill? It's cool to see but think of what happens when people fall while running on a treadmill, and then imagine a poor horse. O_O
This isn't the one I was talking about, but check out his legs as he hits the ground!