I'm a lil late on this one but my horse had a bite like that several weeks ago. I mixed diaper rash cream & hemmroid cream together & slathered it all over the bump. Later that day when I came back to feed it was next to gone. It was undoubtly a bug bite but the hemrroid cream helped heal it up quick.
If she is turned out where there are bushes with thorns, it could also be a thorn. I live in TX now and have some mesquite trees in my pasture and my solid paint filly had a thorn in something that looked like that. (I found out when just running it over with water, and some linement didn't bring it down. poked around and squeezed, the end came out barely, and after some rather uncomfortable squishing for the horse, i was able to grab it and pull it out. It disappeared a couple days later with me washing out the small wound for a couple days and meds.
My horse gets them BUT SEVERELY BIGGER- dont go spending out on expensive fly repellent- keeping in mind these bites are painful get in touch with your local vet for some pain killer and give it to him with the suggested amount in his feed
this is disgusting but works
step 1. find the scab on the bite if you cant find it search more there will be one!!!
step 2. if your horse isnt going to harm you in this doing- pick the scab off and massage the whole spot to soften it up then try and then squeese it -if it pops YAY but if not massage in some soudor creme or something that known for working-making scabs dissapear fast or something like that but i would advise sudor creme
eventually if the creme is strong enough they will dissapear in no time
It does look like a bug or bee sting. Our sorrel mare looked like a pin cushion earlier this year from some sort of bites. They'll go away in a couple days, and I've found that apple cider vinegar seems to help, and mixing it with the fly spray has kept the bugs from biting her as much (they have been bad this year).