Uhm, well, that's a bit of a detailed story.

The short version; Skinwalkers are humans (generally Native Americans, where the tales originate) who can transform into animals, usually by wearing the preferred animals' pelt and/or magics/salves. They are closely connected to Werewolves. The Navajo in particular have strong beliefs and don't like anyone to use the actual name.

However, on various internet forums; Reddit, 4chan, creepypasta, etc. started passing along (and flat-out making up) stories, and the terminology got confused.

Wendigo, skinwalkers, and certain were-animals are all generally considered human...or perhaps, ex-humans.

Fleshgaits were never human.

I have a theory - I've posted it in conversations here on Reddit before, so I'll try to be brief; As humanity evolved over the past roughly 1 million years, another animal evolved to become our primary predator. I'm not saying Intelligent Design had anything to do with it, but that's an easy way to process the information. In fact I don't think Fleshgaits are particularly Intelligent. Not Sapient anyway. Clever, but not Intelligent in the way humans are.

They evolved to be able to hunt, trick, and kill us. Keep our numbers low - because honestly, humans are terrible for the environment and nature in general. Think along the lines as Nature creating a means of culling humanity.

And they did such a great job that we, as a whole, are still unaware of their existence. Just glimpses of things in the wilderness. Basically, all we had for the vast majority of the past few thousands of years are scary stories told around campfires - werewolves, vampires, zombies - basically any story of humanoid creatures that kill humans may trace its history to this race of super-predators.

However, sometime in the past, oh, say 150-200 years ago - roughly starting somewhere between the Industrial Revolution and WWII, the human population exploded! It's estimated that the world population was under 1 billion in the year 1800. In the year 2000 it was 7 billion. In 2020 we hit 8 billion. We've doubled from 4 billion in 1975.

I believe this extreme population growth stymied Fleshgaits. They are no longer able to hide. And with the internet spreading their stories, people are armed with knowledge. Thus Fleshgaits are dying off. The ones still out there aren't as well trained/skilled in their ability to mimic. Especially shapehifting. Most stories talk about how someone hears a noise - a baby crying or a woman in distress, and it's pretty convincing. Like OP's post here. But when someone talks about seeing one, there's often something 'OFF' about it.

So, the good news is we seem to be winning. The bad news they're still out there, and they may be pretty pissed!