There was rumors over the last few months that we would be seeing a large walker type of model released for Space Marines. A source has chimed in that the rumors were "off", and that the large walker we saw at Games Day US was what had been seen.
Please remember that these are rumors.
via an anonymous source on Faeit 212
the big dreadnought that has been seen is not for the 40k sm release, but in fact the big mechanicum walker that was pictured at games day.