Hello,
I actually do know what that is! We had adopted a female a few years ago who had a bump just
like that. Initially, we had put it off but tried home remedies, etc but nothing really worked. It
is most likely going to be an abscess that has been there awhile & penetrated the tissues several
layers deep.
I recommend taking her to the vet for analysis. It will probably need to be surgically removed.
If it isn't too deep, they might just do a local & cut it out. If it has compartmentalized meaning
the infection has affected several layers of tissue & it will be invasive enough to warrant a full
surgery/anesthesia.
Abscesses are caused by bacteria from an area that became infected by either a damaged scale
or other type of trauma to the scale & tissue area.
Tracie