It looks like a blister; it's dark pink and bubble-shaped, protruding from the inside/top of his "knee" on his FR leg. It doesn't seem to be tender when touched, but like I said, he won't put weight on it. He's licking it.
Can I give him equine Bute or something? I hate to see him in pain, however minimal it might be.
Like I said, I'm calling the vet when my mom gets home. Just wanted opinions as to what it might be. Food hasn't been changed; free choice of the same stuff we've been using for the past six years; water available 24/7, never any medical problems before this. I'm going to get my camera to take a picture to show you guys.