I’d say hives. It could be from bug bites or sweating. Or a reaction to something once he started sweating. If it returns, you can use some rubbing alcohol in water and sponge over the areas to help a bit.
If the bumps were soft and not sore they quite possibly could have been bug bites
The only thing I could think of is hives. Was there a change is diet, bedding or quick weather change?
They were all over his body (not face), mostly on his hind. It’s minus degrees celsius where I live, the indoor is heated though. The bumps were soft and not sore. I’m thinking something to do with sweating but I’m wondering if there’s an actual name for it
Was it all over or where the saddle/girth are? Is it warm (weather wise) there? How are the bumps to feel-hot, soft, solid? Are they sore when you press on them?