Nope, I'm thinking Sweet itch. Sweet itch is an allergic reaction caused by Midge fly bites. (Midges are TINY and you don't even know they exist!)About 30 % of horse nowadays have it. Mine does... when it starts to get warmer, he starts to lose chucks of hair like this. He also rubs on EVRYTHING because he itches so bad. Unfortunately there's no cure. AND they can develop the allergy later in life as well as early on. (My horse is 14 and it just started) You can alleviate the symptoms by weekly washings with aloe based shampoo and MTG. That's what I find works best.
Your horse looks just like mine! Have a vet look. I can almost guarantee that's what it is!!