I noticed that a while ago, actually, and again every time I re-read the comic. Vitaly said he writes around 90 pages in advance, though, so I'm hardly surprised.
I believe that plans have changed slightly, though, as they always seem to in comic writing, and that the Lifealope/Catalope was originally supposed to be two headed as shown. Also, about that last unknown monster... Seeing as the Catalope survived the ion blast, I believe that Cancer/Biomass 117 did as well. Perhaps after the Catalope revives Snippy and Pilot it finally gets taken over by Cancer? That's been my current guess. Not quite sure yet, but we'll find out soon enough.
Also, the stick figure with the bow and arrow is probably Snippy, while the other two are likely Pilot and Engie, or Pilot and Captain. (More likely it's Engie, because Captain doesn't exactly seem like the heroic type. |D)