Did you read the theory by princesswoona? It's incredibly convincing:
"I don't think the hooves need to harden or soften at all. Hooves are generally U-shaped, but in the middle is a triangular-shaped soft fleshy area known as the frog. In normal horses, this is spongy tissue that fills with blood that has come down the leg from the heart. As the horse steps, the increase in pressure causes the blood to be squeezed sending it back to the heart by reclaiming some of the force of gravity that is pushing down on the horse's body.
Since Equestrian ponies are smaller, and have thicker legs, I imagine gravity does not become an issue for them, as far as blood returning from the legs to the heart is concerned. This frees up the frog for new purposes, such as a granular gripper thingy!
If an Equestrian pony's frog develops granular cartilage within its volume, and they develop a muscle that pulls the top of an internal membrane that surrounds the frog, then, theoretically, the frog could be vacuumed of pressure, and the granular cartilage could function like the coffee grounds.
Hoof is still hard, but that protects the delicate frog from damage then, as they trot about."