Advice?

[Cinnamontoast]

Within half an hour of leaving the yard, creature had developed a pitted odema on his neck, fortunately not impacting on his airway. The edges are hard, the centre is fluid. Not much heat, not painful (he's a wimp, nearly wet himself when he had his temperature taken!). It's the size of half a tennis ball. Thank God, my mate's BF was there (qualified equine vet, how lucky am I!) I need to check it tomorrow and hope it will go down on its own. Hopefully only a reaction to a bite, maybe.

Has anyone has similar on their horse and how was it resolved?