First, a bit of a history lesson. My lease horse is an ex school horse (20ish year old QH gelding), who I believe they got rid of due to recurring problems in his hind hooves. I jumped at the chance to lease him, and within 2 months he was completely sound, because I had the common sense to try shoeing him on all 4 rather than just on the front.
He's been going wonderfully for the past couple of months. His hooves are almost how they should've been from the start - concave rather than flat, healthy, and his frogs no longer touch the ground. Last week, he started a faint limp on his left fore. We thought little of it because he hadn't been worked for about a week and he was out on the road and rocks. The next day, it was still there, though hardly detectable at a trot. I decided to give him a few days of rest, in case he'd hurt himself (he has a tendency to act like a colt in the paddock).
The next day, another horse arrived where I agist, and my horse was ecstatic to have another horse nearby. He has spent the last few days being a complete fool, pelting full ball along the fence line.
3 or so days on, we come to today. He seems fine walking from the paddock, isn't favouring his leg at all, so I tack up and hop on. He takes 3 normal steps, and then starts limping on the right fore. Now, this is a horse who has learnt how to get out of work by being stubborn and by throwing in an occasional few steps of limping that then disappears, so I decide to give him a lap of the paddock to see if the limping changes. I throw in a few steps of trot just in case, and the trot has horrible limping. The walking isn't too bad, until we were coming up the far side of the paddock and he started limping on the left.
I dismount, untack, and hand walk him to see if there is any difference. If anything, he was limping worse but he was not at all reluctant to walk. Let him out in the paddock, and he limps over to his dinner.
I'm planning to give him another few days off - hopefully the presence of the other horse has lost its novelty now and he might actually rest.
I guess what I want to know is what sort of problems could I be looking at? He has been seen moving perfectly fine at all gaits across the paddock, he isn't being stubborn and refusing to move, there is no heat in his legs or hooves and he doesn't show any signs of pain no matter how I poke and prod.