Seeing that a horse is pretty heavy and can move quite quickly how much horsepower does a hosre develop as it must be more then 1hp

But the heavy ones aren't quick, and the quick ones aren't heavy. James Watt did the original experiments, and while I can believe horses are a bit fitter these days what with growth hormones and so on, the original calibration was against one genuine horse. He harnessed it to a rope, passed the rope over a pulley down into a shaft, tied weights to the end of the rope, and timed how quickly the horse could lift them: 330 foot-pounds per second is what he came up with (i.e. 10lb at 33ft/sec, or 33lb at 10ft/sec). In modern units it's about 750 newton-metres per second, that's to say 750W.

Peter