I've had a world concept banging around in my head for ages, where the basic gist is that the living and spirit worlds are sort of superimposed on one another, but each one's ability to see the other is generally quite limited.
"Souls" are then the spiritual energy of a living creature that's passed on into the spirit world, where its senses dull and its mind gets reduced, as it's not just free-floating life energy trying to remember what it was. Sometimes they reincarnate, looking to become what they once were, and sometimes they just float around and forget what they were doing, either dwindling down and becoming something smaller or clumping together with others into a larger mass.
The reincarnated souls try to find something that resembles what they think they are, and the closer a fit it is, the more "alive" the creature then becomes. But, again, their perceptions are a bit foggy... So sometimes they flop into things that just look "close enough". This is how we get reanimated dead, as the spirits are either confused or tricked into inhabiting bodies that aren't as intact as the idea they have of themselves, resulting in sluggish, clumsy, mostly-braindead animations (this was supposed to lead into a statue created by a master artisan, which was so perfectly lifelike that a soul inhabited it and attained intelligence).
But the soul clumps are more interesting... Now you have a mass of various spirits, each with its own fading memories of what it used to be, but all of them start wanting to live again. Eventually the mass gets powerful enough that it can just kind of muscle its way through and form its own body (or force change on an existing body, causing it to grow into something different), based on what it thinks it is... Which, considering on where the component souls come from, could be quite the combination. This is how we get monsters, boars with fish fins and bears with goat heads. Chimeras. They also tend to be rather large and powerful, if still chaotic and uncoordinated; because even though the body doesn't really match
any of the memories in it spot-on, the mass as a whole has such a large amount of life power that it's basically a supercharged creation.
There's a few holes and gaps in the theme here and there, but it provided a basis for a lot of elements I wanted to include so hah