Hey, OP and Mark--
I have a horse who has a lump on the knee exactly like the one shown in the pictures. Difference is, he had it when we bought him five years ago, and it hasn't changed since. Sometimes it blows up to a big size, like in the photos. Sometimes it shrinks and is barely visible, but it's always there. There appears to be no pattern to its variations. It's not hot and does not bother the horse in the slightest. He is also sound.
Which is just to say....don't know why I'm sharing that story, but it must be something along the lines of what Mark posted, only permanent.