If you are going for a Lovecraftian, Dagon fish demon vibe, then wear something that a fisherman would wear like a slicker, raincoat, big boots etc. But, if it is set in the woods far away from a large body of water where no one would be wearing those clothes, go for ripped up cheap clothes from a second hand thrift store. A straight jacket is a very creepy idea, but there would have to be a reason in the story for the monsters to wear them. Are they escaped creations of a mad scientist? Are they criminal outcasts from their civilization? Or was one kidnapped and put in restraints by regular humans?