Apparently it is nothing. Nothing but a harmless fibrous lump in her rear hind quarter. Vet says she has some others at various locations too. But her rear left leg seemed much bigger than the other (in what would be the thigh area for us). I guess it must have been that way for a while, because she seems fine and the leg still seems to be a bit bigger. Not as much, but just a bit larger. Maybe it has been that way for a while and we just did not notice.