It sounds like a white line abcess (abcess itself probably already gone), possibly following on from shelly hoof. I have had two of my Shetlands with that this winter, my tup is still sometimes lame, sometimes not. I took away most of the horn, did not give any antibiotics (after all i couldn't see any infection), just sprayed with purple spray.
The hoof/horn will grow back slowly, it is probably a consquence of the wet weather with the ground being so squelchy all the time, the have never dry feet and that makes them so soft and easily a stone or similar can cause that kind of damage.