Other ghost pokemon flat out create new ones outta humans.
Cofigirus literally in dex says they turn humans into Duskulls or straight into Dusklops.
Misdreavus is the soul of a decapitated witch.
And that's just the ghosts.
Ninetales is 9 wise men.
Kadabra was once a Stranger Things level pyschic kid.
Marshadow is the spirit of the ultimate fighter refusing to go the afterlife.
All the Kalos swords where once soldiers.
I could go on forever.
They are not dumb animals, and some actually have more intelligence then even the most intelligent psychic human.