Why do horses have skinny legs?

Okay, they aren't entirely skinny -- it's mostly the bottom half, with a big muscle up at the top. But that's odd. Human legs are kind of like that too -- bigger on the top half. Come to think of it, dogs' legs are like that. And cat legs. Wait a minute. Deer legs are really skinny on the bottom and much thicker at the top. Antelopes. Gazelles. Giraffes. Elephants.

No, wait. Elephant legs aren't really like that. Neither are duck legs, or panda bear legs. So it can't be a prerequisite for any leg. What's the use of a skinny leg?

Answer? Running. The skinniest lower legs belong to the fastest runners. Why?

It's because when you run, or indeed walk, you are constantly swinging the bottom halves of your legs. For humans, one foot is stopped and essentially resting on the ground while the other is moving twice as fast as your overall speed. Then that foot stops and the other one accelerates. And so on.

Thinner, lighter things are easier to accelerate than heavy, massive ones. So if you want speed, put the muscles up high and the skinny down low. If you really want speed, you could go even further and pare down the foot, to decrease lower leg mass even more. A few million years of evolution and you'll wind up with...you guessed it. A hoof.