The most important aspect of the health of a horse’s hoof is how it is trimmed.

First, look at the bottom of your horse’s foot (the sole). Draw an imaginary
straight line through the widest part of the foot. From that line back to the heels
should be more total surface area than in front of that line.

Second, the frog should be at least ½ as wide at the heel base, as it is
long. It should also make contact with the ground at the base.

Third, the angle of the hoof should parallel the angle of the pastern. It is
not an exact number but usually falls between 45 and 55 degrees. The best way
to determine if proper angle is met is to look at the foot from the side and place a
straight edge (ruler) along the front aspect of the hoof from the toe to the coronet
band. If it is a correct angle, the front aspect of the pastern will follow on the
bottom of the straight edge exactly all the way to the fetlock.