[jaydee]

I cant see why ant bites as they're a ground living crawling insect and sure the horse wouldn't lie there and allow himself to be bitten so badly, I think you'd see him rolling around trying to get them off
They could be mosquito bites or some other flying insect that bites and tends to travel in swarms
They look really sore so you need to watch out for infection and from stable flies getting into them
Has your vet suggested anti histamines of any sort
There is an ointment you can use that helps healing and acts as fly deterrent that you could ask about

 

[Allison Finch]

I've seen horses get hives like this from fly spray. It happens when people don't shake the spray up well before applying. The ingredients often separate in the bottle and you might put full toxin on, instead of the mixture. The results were a bunch of hives. Once the student started shaking it up well, they went away. Her horse was much more sensitive to some sprays than others.

 

[emilyandgallant]

The vet told me he was rolling in ant hills. It is hard to believe he'd keep rolling in them. I thought they were an allergy to something so I was going to get him a steroid
medicine. The vet told me it wouldn't help... I have been cleaning the sores on his armpits/chest daily with Vetericyn. The sores on his chest/armpits showed up on Monday. It looks like it has slowly started to heal but I don't know.

 

[stevenson]

Yep Ants. Nasty little ******s. This spring, i had 2 pastured horses full of bites , thought mosquitoes at first, put some salve on them, then one day i was out there doctoring them and found ants all over my feet and ankles.. I poured fly spray down the hole. and sprayed fly spray on the ground. It took about a week for the bumps to go away