Hmm. It's funny. The way these "Walkers" work, they might as well be come into play abilities, or sorcerys, since the only time they won't be activated immediately is when your really, really need a blocker. Still, based on the other cards that were in a set, I could see these popping up the way they are written. Plus, a creature that came into play and put another creature into play would probably have to become uncommon, and closely watched. This could probably hang out at common.

@jmg: You think? I think I'd hold this back until my opponent was attacking with stuff before I popped it to cast my Murderous Redcap or Bloodmark Mentor :)

Hmm... maybe uncommon would be a good idea. Thx.