It is on her left front hoof heel bulb. If you follow the groove in the middle of the frog into the heel bulb, that's where it is. Basically, in that area, the heel bulb is peeling away. I spray Apple Cider Vinegar/Betadine mix onto her hoof to treat and prevent thrush, I recently started doing this for the first time a month ago, but I would think it would be on more than one hoof and would have happened sooner if that was the problem. What could it be?? Should I talk to the farrier? It doesn't hurt her when I press it with my thumb. It is about a little more than 1/4 inch. It is not the frog, it is the heel. Thanks a bunch! I will try to get pictures soon.