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

As Peter described, the "1 hp" measure was based on the ability of the horse to do "external" work (pull something), not on how much power it generated while moving at high speed. A horse can develop much more than 1 HP when moving at high speed, but it can't sustain it for long.