It looks like either where an abscess blew out or I've also had horses come back from trail rides with nicks like that from going through a lot of rocks and tree limbs. But if he's not a heavy duty trail horse, I would say those are where abscesses drained.
If he's not lame, I don't see it as a big deal, but something to keep an eye on.
Yeah I rode him through a very woodsy area today and noticed when I got home. He recently healed from an absess but I don't understand why it's on two hooves and why one looks fresh and clean and the other looks like it's healing?