Lamashtu is in many ways the classic witch’s familiar. You’ll find him hanging out with Annamarie Nightshade in the graphic novels, and he also plays a role in New England Gothic. He looks like this not because it’s a fair representation of who and what he is, but because this is what’s expected.
Quite a few things on Hopeless, Maine work in this way, because reality and magic alike are affected by belief and intention. People tend to see what they expect to see.
There’s a case for saying that the island is full of demons. It depends on how you like to define things. If you’ve read Personal Demons (which is in The Gathering) you’ll have met the rather self announcing owl demon. Part of the point with that story is that the obvious demon probably isn’t the only demon. They don’t always turn up looking the way you expect them to.
Evil often isn’t self announcing. Usually, the people perpetrating it firmly believe that they are, in fact, the good guys. If you’re reading the graphic novels, this is a line we’ve explored extensively through the character of Reverend Davies. It would be fair to say that the Reverend always feels like he’s acting for the best and doing what’s right, but sometimes he’s alone in those beliefs and his actions are, in practice, hideous.