My horse made a nice run and after cooling him out I let him drop his head to graze while i untacked. I immidiately noticed just one nostril was coated in blood. I have no experience with a horse bleeding out.  I realize many do bleed whether it shows externally or not.  It was just the one nostril and it was not a lot by any means. I rinsed it away and it wasn't continuing to bleed. 

I will call in to the vet this am for advice but many here deal with this on the regular so what does it mean for him and what do I do now?   

There are a ton of threads on this. I used to own a bad bleeder. Bleeding out of one nostril was normal, coughing and head shaking after a run are also symptoms. Whenever my mare would bleed out, I gave her some uniprim to prevent any infection and some time off to help heal. The main thing in taking care of a bleeder is run them on a preventative – Lasix, bleed stop, Equipulmin, etc. Also make sure they are in tip ton shape.

Bump

My horse did the same thing.  Just one nostril.  Only after he had his head down eating.  We ran on lasix for a few years. He doesn't tolerate lasix well so he no longer 

runs.  We tried so many things.

My vet explained it to me like this.  Look at how huge their lungs are and how much blood and pressure it would take for the blood to go uphill

and come out their nose. You may only see a little but there's alot more you didn't see.  My horse ended up getting an infection in his lungs while on lasix.

Took a long time to clear up.  Wasn't cheap.

He said that most horses bleed from the base of thier lungs.  That was true of mine.  You could see scar tissue in the base of his lungs.

Who knows how many times he bled before I saw it.  He never faded, never coughed or shook his head. Never acted like a bleeder.

He had every possible lung exam, ultrasound, xrays, scopes, brachial lavage, pulmonary funtion test.... Never could see a reason he bled.