How to deal with fear of this dog?

There is no way to know if this dog is aggressive or not. And, your Mom is a little foolish taunting the dog since he may dodge her "Boo!" at first only to come back at her and try to bite since he might feel threatened. Your fear of this strange dog is NORMAL and WISE! And, there is no way I can help you lose fear of something that you SHOULD have a bit of fear from. Your feelings are your feelings.

But, as others have said, you should not have to worry about going into your own yard for fear of being bitten. Call Animal Control. Not only for yourself, but for the dog's own safety as well. As you are afraid of the dog, others might be as well, and he could easily be shot for acting aggressive to other neighbors, or be struck by a car, of cause an accident, or injure a child. For all these reasons you should call Animal Control.

Having some fear of an unknown potentially aggressive dog is a very HEALTHY way to feel

Stay away from it and seek therapy for yourself.

Just stay away from the dog. If is is loose on the street/road, call animal control. If you know the owners, just take the dog home or call them & tell them their dog is out again.

Then steer clear of the dog - there is no reason you need to interact with other people's dogs in your neighborhood.
If the dog is running loose - report it to animal control and have it picked up.

If he seems particularly violent, call animal control! Otherwise, just ignore him.

You should probably figure out what breed the dog is. Once you do that, you should research the dog's breed. You might find out that dogs of that breed are typically friendly. If this doesn't help, I suggest having someone with you whenever you are near the dog.

Get a puppy and raise it and you'll realize dogs love people and bark because they are excited