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.
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