Actually ... I think they came from the new fantasy Necrons, just look at some of those new beasts they are creating! Eventually the fantasy Necrons develop their new creatures to the point where they have there own zombie like brains, and these germinate into pre-tyranids, and those of course evolve into the Tyranids. One or the other rebels against the skeletal Necrons, and these evolve into the Necrons with their metal exo-skeltons, or whatever ya want to call them. Shocking, I know.