You'll hear horse people talk about a horse's frog. What is it?
A horse's hoof is basically its toe nail, wrapped around the outside of the foot.
The concave area is the horse's sole. The triangular shape is the frog. It's a spongy structure that acts as a shock absorber, kind of like a dog's paw pad. Note the deep groove on either side - this is where stones and other objects can get stuck, especially if the horse is shod. The frog should touch the ground in soft footing - if it doesn't, the horse can actually develop circulatory problems.
Why is it called a frog? No clue. The etymology of that term appears to have been lost.