The 17 yo TWH I got a few weeks ago had large, thick "dandruff" in mane. I got her a few weeks ago and her background is that she was rescued in July from a situation where she was basically in the woods starving. Since I got her a few weeks ago, she has been taking supplements from SmartPak -- including Omega Horseshine, SmartHoof Ultra, MegaCell, and some others. So, lots of things to help skin / coat / hair / hoof / etc.
Anyway, about a week ago, I got her main and tail all combed out using Cowboy Magic Detangler. Then, yesterday I noticed that in a line along her mane, she had these huge chunks (nickel and quarter sized) of thick dandruff -- really looked like thick "cradle cap" that babies can get (or a thick scab, but it definitely wasn't a wound scab) and looked like a thick layer of it had detached from her skin and was growing out along her mane. It was gross, but the skin underneath looks good to me -- no crush or flakes (though I am a complete beginner). I combed all of the scabby cruddy stuff out with a comb and gave the area underneath a good massage with a mane / tail "brush" (http://www.amazon.com/Oster-Equine-Care-Mane-Brush/dp/B0006G5NQY ).
My thought is that the supplements are taking effect causing her mane to grow and the skin underneath to be healthier and thus "shed" off this layer of crud. Does this make sense? I don't really want to add harsh topical anti-fungal / etc. treatments if her body is naturally healing itself, but I am glad to do whatever she needs. Has anyone dealt with this before?