Hmm I was thinking strongyles, but maybe it is hair, that'd be weird though... it's just turning autumn there right? So no massive winter coat shedding.
I'd say take a sample to the vet.
If it really IS hair, maybe give 'em a red salt/mineral block, see if your horse was eating hair and stuff to try to get some mineral he needed by eating non-food stuff?

I'm not sure how he'd accidentally eat hair. Seems like they are very particular and good at sorting things they don't want to eat. Mine can grab a mouthful of tiny mixed plants and spit out whatever he doesn't want.