ok there are nosebleeds and NOSEBLEEDS!
nosebleeds are fairly common in high performance horses after exercise. This condition is called EIPH (Exercise Induced Pulmonary Haemorrhage). The kind of horses affected are very top class eventers and racehorses, after a very hard workout. There is usually little blood at the nostrils which sprays out when they exhale. It stops very quickly and is nothing to worry about.
However, nosebleeds in horses can also be caused by any of the following:
1. Head Trauma
2. Infection (sinuses)
3. Nasal masses
4. Guttural pouch infections (VERY SERIOUS)
5. Lung abscesses or pneumonia
6. Fungal infections
It is quite important that, if the horse repeatedly has nosebleeds, she should be examinaed by a vet as some of the above conditions can be very serious. However this should be taken into context with the amount of blood and how often it happens. Occasional small nosebleeds are ok but regularly they are not, regardless of how much comes out. It is possible for a horse to bleed to death if it has a guttural pouch problem causing haemorrhage, as one of the major arteries to the brain runs through this area, and becomes damaged by infection. This results in a few small haemorrhages followed by one or two massive ones. NOT a pretty sight believe me.
There are some things I'd watch and see, but nosebleeds are something I do think should be investigated properly f you are worried.