In the end I brought McMug to the vet. This rascal is now afraid to go to the vet. He would stopped walking at the steps leading to the clinic and I had to drag him in. He wouldn't sit still while we wait for his turn. He whined, went up and down the seat, squeezed between my back and wall, and paced around the waiting area. He even growled at a Shetland who tried to be friendly. Hmph. Grouchy ain't him?
So the vet saw that ugly lump and thought it might be some sort of sting by bee or centipede. I had to hold him still while the vet assistant tried to squeeze the pus out. McMug was given antibiotic, anti inflammatory and anti-fungal medication, and I was given a $69 bill. Urgh.
And what's McMug doing now? lying on my bed with his trumpet looking like "E-collar" on. We had to put that on for him to prevent him from biting or licking the lump. Don't worry, McMug is still as active and as much a rascal as a Jack Russell can be.