Why would there be a horizontal crack in the wall of your ponies hoof?

this has happened because you havent been moisterising your ponies feet before hacking and training.

you could try these:

1. letting your pony have a couple of days off while you take care of his hoofs.
2. carefully pick out for ponies hoofs everyday before schooling and hacking
3. after you've finished picking out for ponies hoofs, put hoofs oil on the hoof wall
4. If the cack dosent desapair in a a week or so, consult for vet

good luck!