Maybe Mew was the 150th pokemon at that time. They seem to know about it, too, so before Mewtwo was created, Mew could have been the 150th pokemon...but then it wouldn't make sense for Mew und Mewtwo to switch places...*shrugs*Kyoki said:In the sky above Johto? I don't know where exactly though.
Also, I've been thinking of this for a long time. Remember when Red said that there were 150 Pokemon known so far (vol. 1)? How did he know about Mewtwo (or how did anyone else know about it) if he didn't really know anything about it?