I wouldn't feed fish oil to a horse since it's an animal product and therefore not in a horse's natural diet. If I wanted to up the O3s in my horse's diet I would give something that is plant based like flaxseed.
I do the same with my Labs. Since flaxseed isn't something natural in a dog's diet, I feed them fish based O3s like sardines in a can or fish oil.
I will not give Cod Liver oil to my dogs since the liver is where all the toxins are processed.
"When you're young and you fall off a horse, you may break something. When you're my age, you splatter." -- Roy Rogers