Join Date: Mar 2009
Location: Washington, USA.
As was mentioned, it's most likely an abscess. Whenever a horse comes up mysteriously lame, most every horse owner/person I know will soak the hoof in warm water and epsom salt. If it is an abscess, he will not react to pressure from using your fingers to poke him. You need hoof testers.
If it's not an abscess, my next guess would be he stumbled and tweaked his fetlock. In that case, cold hose or ice it for the next day or two [remember, 10 minutes on, 10 minutes off, 10 minutes on, etc, NOT thirty straight minutes] and give him some time off. When he's sound again, wait a few more days and then start him back in work.
I'm sure he'll be fine, but it's good to have the vet out anyway. As iride said, with time you'll learn when you need a vet and when you don't, but until then, it's best to call one out. =]
"Great spirits have always encountered violent opposition from mediocre minds."