That convenient gap between the front and back teeth, which allows for the use of a bit, probably resulted from the evolutionary lengthening of the horses face. A long face allows the eyes to remain above the grasses while eating and see predators farther away. He can see almost 360 degrees around him with only a 3 degree blind spot at his tail. The horse's depth and color perception is worse than humans but his night vision is much better. This is made possible by the efficiency of the different gaits a horse is capable of.
Horses can produce five times the volume of sweat per area of skin that a human can.
That's important because horses cannot breath through their mouths and therefore cannot dissipate heat by panting. Despite considerable effort and money spent improving race horses' speed very little improvement has been made in the last 100 years.