I was wondering what they are, as I have heard the term used before. I know it has something to do with the angle of the heels being less than the toe, but could someone please elaborate, or post comparison pics? Thanks!
Lioness did a a great job of demonstrating things, but I will correct one thing. A horse will always have heels, underrun or not. Underrun heels does mean a horse has no heel. It means they have too much heel and it's grown outward with toe rather than downward like some horses. My yearling for example will get underrun heels if I do not trim him every week because his feet grow that fast. My pony on the other hand also grows a lot of heel, but has very upright feet so they grow downward and give him tall heels rather than underrun heels. Posted via Mobile Device
This is my yearling(his feet are a work in progress!!) This is just less than 2 weeks since his last trim. His feet grow forward at a great angle.
And this is after.
Voila! He magically has a heel. It's always been there, but I trimmed off the excess and brought it back under him. I need to bring it back a touch more but that was all I could do yesterday evening. His feet just grow so dang fast. I obviously cannot let him go more than two weeks without trimming, let alone a week without at least rasping his heels back. Lesson learned for me.
Its just an incorrectly managed foot. Any horse that tends to grow hoof forwards, instead of down is very prone to this, but I think most horses, if trimmed poorly enough, can develop these. When I got my arab mare, her feet looked like this:
Turns out she was bad with her feet, so he left her all winter without taking the shoes off or trimming her. She actually has very upright feet, but with shoes restricting her hoof natural expansion and left on way to long, they looked like that.
On the track many people still believe that increasing toe length slows break over and increases speed, so they trim them like that regardless of long term soundness.