In the setting I'm building, then there are two lifeforms to have existed; Angel and Demons. Both are closely related, as speciation between happened rather recently in their history. They both also have very different forms to other animal species; they have physical bodies, but they're amorphous in the same sense that fog is amorphous. They're actually meant to look a bit like fog themselves; the angels White fog, and Demons black fog.
Some things that they're expected to do is to fight, though how does one fog cloud fight another fog cloud? They also had spliced their DNA with Humanity's ancient evolutionary ancestors, resulting in the Nephilim and Cambions, who were able to revolt and kill off their creators.