I just noticed one of these on Cabra's knee.
My dobermanns used to get hygromas on their elbows and sometimes hocks from the rough way they would lie down on concrete floors. They would flop down and thump the elbows and one of these would form.
Since he's now sleeping in the chicken's empty feed bunk, rather than the tanbark or in the hay, I figured he smacked it on the bunk, either getting in or out.
I have this stuff that we put on the dobies called "Iodex", a nasty black, vitreous, gunk. It's one of those "voodoo" things that reading the ingredients shouldn't do anything, but seems to work. I have some DMSO for the horses, but it completely slipped my mind. I will use that.
There is no heat, bruising, broken skin or anything but the soft, discrete, swelling in the joint that came up overnight. He has no pain, either. Vet said she thought it was one, as well.
Thanks for the heads up about the DMSO, totally spaced it.