The hind leg is the engine of the quadruped. When a draft horse pulls a heavy load, the read end drops lower and the legs develop tremendous power.
The front legs pull hard, too, and they are important in turning and in leading off gait changes.
The rear legs seem to play a key role in changes of forward and backward direction.
Here's a brief film clip of the 2-week-old filly Flashy Princess. She gets confused about the barn gate and doesn’t know which way to go. If you watch her hind legs, they're doing a lot of work has she changes her mind about going forward and backward.
I'm trying to learn more about this, so I’d be interested if any animators or horse experts among you can offer your thoughts about the different roles of the front and rear legs in a horse's movement.
If you have trouble with the embedded video, try this link: http://www.youtube.com/watch?v=TvHOFUAKt1k
By the way, thanks to Irene Gallo for mentioning my upcoming book in her blog. Check out her post here, and if you don't know about "The Art Department," it's a must read for anyone in the fantasy and science fiction art field.