actually no, technically one hp is tha amount of power used to lift 33000 lb one foot over 1 minute.

"To help sell his steam engines, Watt needed a way of rating their capabilities. The engines were replacing horses, the usual source of industrial power of the day. The typical horse, attached to a mill that grinded corn or cut wood, walked a 24 foot diameter (about 75.4 feet circumference) circle. Watt calculated that the horse pulled with a force of 180 pounds, although how he came up with the figure is not known. Watt observed that a horse typically made 144 trips around the circle in an hour, or about 2.4 per minute. This meant that the horse traveled at a speed of 180.96 feet per minute. Watt rounded off the speed to 181 feet per minute and multiplied that by the 180 pounds of force the horse pulled (181 x 180) and came up with 32,580 ft.-lbs./minute. That was rounded off to 33,000 ft.-lbs./minute, the figure we use today." -- http://www.web-cars.com/math/horsepower.html

It would depend a lot on the horse. Of course we're probably going to use thoroughbred racehorses for our horse/bicycle speed tests. Although a clydesdale ought to be able to put out some good speed if we use some really high gearing to make use of his high strength.