Dogs don’t have the best eyesight and may recognize you by certain characteristics, and a beanie may throw those off. Dogs can also be a bit territorial at times, so if this is a problem, maybe try taking the beanie off before your dog sees you and putting it on in his presence. I had the same thing happen as a kid - I got new glasses and my cat hissed at me.

Dogs do not have normal human level eyesight. They rely a great deal on a super sensitive nose… One time just after dark I was putting some garbage into the outside bin. Our dog, an Australian Shepherd, happened to be outside and came up and was growling at me. I saw him in the dim light but he didn’t recognize me. I talked to him and slowly held out my hand for him to sniff and he calmed down… Not all creatures experience their senses exactly the same. Dogs have relatively poor eyesight and I am guessing the beanie confused your dog or maybe your clothes had an unusual odor that contributed. I think the best thing to do in such a case is to talk to the dog in a friendly tone of voice and move slowly. Ideally they will recognize your scent.

Some dog's really hate hats.