What Yuki-onna said.

Except I also believe a lot of it does have several origins. Werewolves in all their forms as an example. Several places believed in them, yet all had different beliefs as to what the werewolf's abilities, attributes and anything else actually were. Some believed they had some weaknesses that others didn't, some didn't see them as necessarily evil in any way.
There's a lot to it...

Some may have become a little more uniform with some globalisation, but time played a big part too I'd hazard to guess.