Could it be a gravel?
A "gravel" is a condition where an abscess, instead of coming out through the bottom of the foot, works its way upward beneath the hoof wall and breaks out at the soft tissue of the coronary band, where the infection drains out. It is not, however, literally a piece of gravel working its way up the hoof. "I find that ‘gravel’ is a regional term for describing an abscess," Bill Reed says. "The farther north you go, they say that a horse ‘graveled’ rather than ‘abscessed.’ "
According to John Burt, "Gravitational force and the way that the foot is constructed make it questionable that an actual piece of gravel can travel up there. Look at the structure of the foot and figure out the gravitational force: It pushes down. When an abscess is created, the pressure is pushing it up the foot."
He reasons, "If you did find a piece of gravel in an abscess, it was driven in from the bottom, and then the abscess itself pushed it up through the coronary band."
In riding, a horse's energy is like a river- guided by the banks but not stopped by them.