Join Date: Apr 2008
Location: Gallant, Alabama
That looks like a bad saddle fit.
My own mare has spots on both sides of her withers, worse than that, from an ill saddle fit from me using a synthetic saddle on her during her pregnancy. The saddle stretched out and gave her nerve damage, thus the white hairs.
I'd check your saddle-fit, just to be on the safe side.
Horseshoe Loop Farm: Home of Gypsie (1994 TWH mare), Dakota (2006 TWH gelding), Jo (2012 TWH filly) & Minnie (1992 Paint mare)