I'm not a vet but if it were my horse, here's what I'd be thinking. I'd be worried if it was more than trickle of blood, and if it hadn't stopped fairly quickly (15 minutes is about as long as people's nosebleeds) I'd definitely call the vet. I would watch like a hawk to see if it happened again and call the vet again if it did. Also, if it bled from the other nostril I'd be more worried as well. If there were swelling, anything other than a slight clear discharge, etc., I would also be more concerned.