Well, actually, I feel pretty qualified to answer this. I've had three mustangs and many Throughbreds and Quarter horses. The fastest horse in these comparisons is the Quarter Horse. The Mustangs are very sturdy horses. read more
In a short race a Quarter horse blows away a Thoroughbred, Quadruple the length of the race and the quarter horse looses. Quadruple the length again and the Arabian beats the Thoroughbred. Quadruple the length one more time and most Western Mustangs(wild horses) beat the Arabian. read more
EDIT: Ray Douglas points out that the Australian horse Black Caviar ran faster than Winning Brew. However, Winning Brew is still the fastest Anerican horse. More information about Thoroughbred speed records can be found here. read more
Horses can run pretty fast. They can go up to 45-55 mph if they really go at. In fact, the fastest Quarter Horse ever was clocked at 57 mph. read more