In our solar system which other planet could have been in the "habitable zone" in the past?
This answer is also speculative, but according to the planetary habitable zone models proposed by "Habitable zone for Earth-like planets in the solar system" (Franck et al. 2000), a key finding was that:


  an Earth-like planet at Martian distance
  would have been habitable up to about 500 Ma ago while the position of Venus was always outside the habitable zone.


So, Mars could have been habitable, but as it is much smaller than Earth, it has a smaller gravitational field presumably a major factor in how it lost much of its atmosphere.