What Is A Horse's Frog?

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.

Image source: Alex Brollo via Wikimedia Commons. (Excuse the annotation, it was the only pic I could find - if anyone has one they could let me use...otherwise I'll have to stop by the barn with my camera).

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.