That screams LICE to me :o Look really closely around that spot and under your horse's mane and at the base of the mane... they aren't contagious to humans, but icky and uncomfortable and can lead to worse skin infections due to itching themselves raw.
We had a miniature horse that came to our farm who we discovered had lice... ick. She had ONE single bald spot on her neck, and upon closer inspection, the "dandruff" was moving in some places. The only place we could actually SEE that it was lice was around the bald spot :P and, your horse's "dandruff" looks just like what ended up being the lice and the castings, etc. Gives me the heebie-jeebies just thinking about it. We had a vet out right away, and ended up shaving the poor mini, bathing it half to death, and then powdering her with the horse-specific otc lice treatment. Basically, other than the OTC stuff and bathing/shaving there isn't much you can do about it... but, what you can do is pretty straightforward, simple and relatively cheap in the grander scheme of potential diagnosis! We then re-washed and re-treated our mini two weeks later. Never had another problem.
There are two kinds of lice horse can get - biting or sucking. On that note, it can also cause anemia, depending upon the type/severity of the infestation. Ick. Ick. Ick.
Last edited by sillyhorses; 02-09-2013 at 11:09 PM.