we have ruled out pigeon fever, its firm & hard in some spots a big lump.
Thinking she rubbed it or an allergic reaction to a bug? Thoughts?
I’d suspect gnats are biting, causing itching and irritation, and the horse is biting the area to scratch. It looks like some of the midline further back is also bare? Gnats do that, too.
My go to for this is zinc oxide slathered on the midline & any other problem areas. Maybe a topical steroid under the zinc for a few days for anything look looks really irritated.
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No clue but I’d probably have it biopsied and depending on what came back treat it appropriately.
In the mean time, I’d slather it with Coat Defense paste or diaper cream.
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Thank you! I was thinking some type of bug!
I think bug issue is a fair place to start.
If it does not get better with normal care then call the vet out to have it looked at.
The gnats have been HORRID this year! I have one that goes out with a white stripe from jowls to udder every day. Some horses are just really bothered by them 
Lemon eucalyptus can apparently help with gnats, but I’ve just not found it effective. The zinc oxide provides a barrier, and helps the skin heal.
Hope this resolves for you soon!
Agree with others that this looks bug related. My horse has identical looking patches in both of his armpits. If I’m religious about using SWAT, they go away in a few days. Worth a try before having a vet out.
Thank you! It is right next to her armpit! It’s firm in some spots which I think is from biting at it! Here’s another picture from a distance to kinda show where it is and how it looks, I found it two days ago and it’s gone down quite a bit in size from cold hosing, I have been putting Fly SWAT around it!
It’s possibly from her itching it. My mare uses her teeth hard and has created this a couple times.
I will say MTG got us all under control. So easy - albeit stinky. No itching and she’s so much happier.
Thank you for the response!
I did coat the swelling in Sore No More poultice to start to calm the irritation and reduce the swelling. I applied it twice a day and it went away within a few days as I recall?
Then after finding here peeps talking about MTG to repel the gnats and reduce the itching I started that and all our problems went away. Pretty amazing how her violent itching stopped.
I’d bet an abscess that has resulted from a bug bite.
I brought my horse in to the barn this past July after riding her, and as I sponged her off, the sponge came back with some blood on it. I looked and between her front legs there was a spot that looked like it had a hole in it that was bleeding, not profusely, but definitely fresh blood coming out.
I freaked a bit, thinking that it some kind of puncture wound or something. I called the vet and she came out a day later. She cleaned it out well (used betadine first and then followed by cleaning it with Hibiclens), and said it was an abscess (that had opened up and drained) that resulted from a bug bite.
I had never had this happen to a horse of mine before.
The area around the drainage site remained sort of hard to the touch for several weeks after the wound closed. (The vet didn’t do stitches or anything like that, just said to clean it daily and put topical antibiotics on it). The whole area was a little less than 3 inches in diameter.
