Quoted By:
I think that ghost pokemon are just creatures with bizarre powers. Many seem to be multi-dimensional being (able to pass through walls, some are privy to other realms like the spirit realm or the Distortion World, shape-shifting and mimicry). They may have a fascination with death and are drawn to places of death. Humans just attach superstition to them because they are so otherworldly.
I have a certain headcanon involving Gastly. I think that some Gastly may be born from the gases released by decomposing bodies. Humans seeing Gastly suddenly rising from remains might believe that the Gastly was the spirit of the deceased.
Similarly, I think Banette have a fondness for junk, so they hang out in junkyards and play with what they find there. Their fascination with trash might be why their mouths are zipper-like. They like to mimic zippers (their mouths are not actual zippers, if you pay attention to how the "zipper" opens in Pokedex 3D and in the anime). A zipper may be amusing and absolutely amazing.
