This is another one of those subjects that it's good to know. Honestly, I can't remember if I've posted about this before or not because I have dial-up now (oh, boy, are you going to read that refain a whole lot from now on...) and it's hard to do searches if I want to get anything typed and not spend all my time waiting on something to come up.
http://www.ask.com/bar?q=post+legged+horses%2C+dropped+pasterns&page=1&qsrc=0&ab=2&u=http%3A%2F%2Fwww.horseshoes.com%2Fanatomy%2Fesad%2Farticles%2Farepostleggedhorsesdsldcarriers%2Farepostleggedhorsesdsldcarriers.htm
(If this link doesn't come out, then google or ask.com 'post legged horses, DSLD' and it'll come up.)
Postleggedness (I don't know if that's a real word but it is in this blog) is something that was brought up on a horse board I'm on. I'm against a post legged horse in the first place. The dropped pasterns I'm definitely against and will reject a horse solely on that criteria, even if everything else is just what I want. I don't need the problems that will ensue.
Post legged came into being because race horse breeders/owners/trainers figured out that this type of leg could propel a horse faster down the track. Not as much movement sideways, all movement was forward. This post-y stuff has even shown up in the show ring in WP horses (Western Pleasure) for some dumb reason. But the post leg doesn't have enought to step under the horse and get in the ground for, say, working cowhorse events.
At any rate, this is something that shouldn't be bred for but no one is going to stop doing it, especially racing. Too much money to be made. I'm a capitalist and want to make money but I feel/know that intentional breeding of really BAD conformation, no matter the purpose (so what if we have a little bit slower race horses...or is that just naive on my part??) is bad for horses overall. I'm against breeding for this hind leg.