Good question! [he says with a laugh.] No-one really knows!

[a beat:] We do know most of the ghost types used to be human, though. Niko's Gengar, for example! Back home, wild Gengars are infamous for suckin' souls out of people who travel alone at night because they want a traveling companion.

[he says this like... he's talking about a funny anecdote. it's a cheerful hellworld they live in, alright.]