What the heck is this!? Tick? Tumor? Wart? Growth?

[FlyingNunFarm]

Looks like a big chicken black head. I would think puss would have been yellow and harder. That looks like sand packed in a pocket.
I agree regular flushing to make sure it doesn’t fill back up is needed.

 

[Eggcessive]

I still think that it could be an epidermoid or inclusion cyst. Sometimes they need to be removed or to get the cyst out. Since it is next to her preen gland, she may be constantly irritating it. I would try to keep it cleaned out, and the yellow material is probably dried sebum or waxy material. Here is some info and a picture about epidermoid cysts and a video of removing one:
https://www.hxbenefit.com/epidermoid-cysts.html