Why is my horse terrified of large dogs wearing white T-Shirts?

[Ava]

Sounds crazy, huh? I've owed Sir-Rad since he was a colt.
He's 21. I own large dogs and over the years one of them has had surgery, and I've put a white T-Shirt on the dog to cover the incision. Today my Golden has to wear a T- shirt to cover a cyst removal. If Sir-Rad sees him he becomes frantic to the point he could seriously hurt himself.
I've solved the problem by putting a darker colored T-Shirt on the Golden, but I'd like to know the reason why my funny little horse goes bonkers when he sees a big dog in a white T-Shirt?

My vet doesn't know.

[lmattl]

That strange. I would say maybe a big white dog bit him, but you say you have had him since he was a colt. Beats me haha