Let me start with the fact that he's NOT my horse, so advices on "call the vet" are appreciated but useless. It's up to owner (not me), so I can say it, but I can't make it happen.
I was in ring yesterday watching my next-next neighbor riding, and I noticed the horse is dragging his back foot (just one). I mean really dragging so you can clearly hear very loud "shuuurh, shuuurh, shuuurh" kinda sound when he drags it on blue stonedust arena cover. He doesn't look lame, and he still moves pretty decent, but it's very weird. The neighbor said he "rides out" of it later in ride (may be after 10-15 mins of trotting). Well, my qh used to drag all her 4 feet out of laziness until I started dressage on her (so she has to move energetic ), it's definitely not a case here though. He's 11 years old OTTB.
So anyone run into something like that before? Could it be arthritis? Any suggestions on what can be done?