I personally have had abscesses just like this, where once most of the pus comes out, a little blood comes out too. For myself I drain it daily without pressing too hard. It should heal by itself over time. It is generally a staph infection and reptiles and amphibs carry relatively larger amounts of staph than a human.

They are basically a skin infection and can be surprising large, and go deep into the skin. I once squeezed one too hard and it burst under the skin. I then had to go to the ER to have it lanced. It was on the side of my chest. When they were done cutting and draining they cut completely through all layers of skin. What was left was a hole so big I could drop a nickel through it without touching the sides of the hole, and I could see the muscles on my ribs!

(I thought it was neat, because I like medical stuff.)