Uh-oh. Sounds like lice. A couple of horses on our yard developed this over the winter and all the neds have had to be zapped with louse powder. Typical symptoms are patches of bald skin. My vets said you can get many different makes in any good tack shop but the best is Battles (even though it may say on it do not use on animals!) I'm just about to Google it!
It could be that it's just wearing the hood that has caused this. My daughters cob only wears a combo or any sort of neck cover when its really foul weather as his mane will become thin. I think they just become too warm in this area. If he has to have a hood on spray some mane and tail conditioner on to help the fabric slide over the hair. Really it would be better to leave it off, let the air get to it and I'm sure his mane will recover.
Tarquin's lost half of his mane, despite his combo being silky with no seam half way up the neck. Agree, they probably get hot here and it comes out, I've just given up with it. He defo doesn't have lice and I personally never put mane and tail conditioner on his mane as I had an accident on a racehorse where someone had thoughtfully done this and it made my reins so slippy that I couldn't stop!
I agree with Thistle, blame the rug!! B's old rug thinned her mane out and Berry's has also been thinned out - but then they both have loads to spare! Only at the base of the neck mind, not the whole mane.
I sew a bit of fibregee in all the rugs I have just where they will rub, for those without a neck i leave half of the fibregee exposed, if that makes sense???