Conformation

Equine conformation evaluates the degree of correctness of a horse's bone structure, musculature, and its body proportions in relation to each other. Undesirable conformation can limit the ability to perform a specific task. Although there are several universal "faults," a horse's conformation is usually judged by what its intended use may be.

Here's some examples of conformation faults in the legs.

Also, here's a link i found that may help evaluating conformation:

http://www.caes.uga.edu/Publications/pubDetail.cfm?pk_ID=7996