Owls can get pretty terrifying when they feel threatened or when they're pissed off. I wouldn't want to be alone with one.
Exhibit A- Imagine this suddenly appearing before you on a dark road, with the eyes reflecting red.
Exhibit B- Owls can make themselves look considerably more evil when they want to.
I don't know that Mothman was an owl, but I guess I wouldn't be surprised. An owl would appear threatening and could divebomb people. An owl could have perched on the Silver Bridge. That place is all forest, I bet there are owls everywhere, and it's creepy enough in Point Pleasant that anything threatening would be way amplified.