Tail Lump: Final Diagnosis

My horse, the medical marvel. Here's what the final pathology report says:

ADDENDUM
The following histochemical stains were performed on haired skin, ventral tail nodule (slide 1; 4 sections): 

Toluidine blue (mast cells): In 2 sections, there are small aggregates of mast cells containing metachromatic granules. 

Throughout all sections admixed with the eosinophils there are moderate numbers of individual mast cells. 

Comment: 

Histochemical staining with toluidine blue reveals the presence of clustered mast cells, consistent with a diagnosis of cutaneous mast cell tumor. Serial deeper sections did not show evidence of Habronema parasites or any other additional findings. Equine cutaneous mast cell tumors are usually benign and often, as in this case, very eosinophil rich. Complete surgical excision is curative. Anecdotally, in some cases even partial excision has resulted in spontaneous regression of the mast cell tumor. 

Amended morphologic diagnosis:
Haired skin, ventral tail nodule: Cutaneous mast cell tumor

FFS.

If you remember, this was the least likely of the three original possible diagnoses.

What a special snowflake.

Waiting on the vet's word, about whether excision is the next step and if so, how soon. Given that it is definitely receding thanks to the bioabsorbable cisplastin beads we sutured in this may not be an immediate step.

Either way, seriously? I have been exposed to my fair share of medical weirdness in person, and I am an avid reader of the COTH forums for weird medical stuff, and I have never even heard of this. I even did a search for "mast cell" on COTH and it came up with cancer stories in people and dogs. No horses.

oy.