So I was picking out Ice's hooves today, and I noticed that he had a piece of bedding stuck in his frog. I pulled it out, which didn't seem to hurt him, and I was just wondering if I should be worried or not. There wasn't any bleeding, it was just a little hole maybe 1 cm deep and probably less than 1 cm across. He wasn't lame on it as far as I can tell, and he didn't flinch when I pressed down on it.
I agree that it may be from an abscess that's already burst. It could also be from thrush, or depending what you mean by 'hole', could be a crevice due to his heels being contracted. Looking at hoofrehab.com, among other good sources, will give you a bit of an idea what you're looking at & how the hooves function, in order to work out what may be going on & what is needed.