I have a long yearling colt that seems to have been born for the Western Pleasure show ring.  However, he cut himself badly a while ago, and it's going to leave a pretty big scar on his leg.  Will that affect winning in the Western Pleasure show ring?  Also, how do I go about training him, as far as groundwork?

A blemish has no bearing on a performance-based class, as long as it doesn't impede his ability to do his job.

 As far as groundwork, you can do the basics like training him for any other discipline.  Get him used to the idea of lunging, wearing a saddle, giving to pressure, etc.  It's not until you get past the basics under saddle (like, whoa, go, and turn) that the training starts to get discipline specific.

Thank you so much!!  I am SO relieved that his blemish won't affect him in the ring (it's not going to make him lame, but it's not going to be a pretty scar)!

Thanks again!!!!

 Yes the only thing he may not be good in is Western Halter. But yes he should be good for Pleasure. If he can hold himself nicly then yes he will be a good western pleasure horse. 

 Here is some of the stuff I got straight from the AQHA Site:

" (a)Western Riding is an event where the horse is judged on
quality of gaits, lead changes at the lope, response to the rider, manners
and disposition. The horse should perform with reasonable
speed, and be sensible, well-mannered, free and easy moving.
(b) Credit shall be given for and emphasis placed on smoothness,
even cadence of gaits (i.e., starting and finishing pattern with the
same cadence), and the horse’s ability to change leads precisely, easily
and simultaneously both hind and front at the center point between
markers. In order to have balance, with quality lead changes, the
horse’s head and neck should be in a relaxed, natural position, with his
poll level with or slightly above the level of the withers. He should not
carry his head behind the vertical, giving the appearance of intimidation,
or be excessively nosed out, giving a resistant appearance.The
horse should have a relaxed head carriage showing response to the
rider’s hands, with a moderate flexion at the poll. Horses may be ridden
with light contact or on a reasonably loose rein. The horse should
cross the log both at the jog and the lope without breaking gait or radically
changing stride."