You are on to something there, Infoceptor--the game could definitely use an equivalent to the barbarians of the Civilization series. Space pirates would probably be the best bet for that, though.
As for the space monsters: they're one of the bajillion game features that were left out in the end, but still have remnants hidden in the XML. You know, along with Terror Stars, Employment, Antimatter Power Plants, the Starship Foundry, Pain Amplifiers, the Historical Preserve, Life Force Power, the Starbase Upgrade Center, Cabinet and Government "abilities", Crime, ships docking at Starbases, Upgraded Starports, some kind of S_Omega XML tag for planetary improvements, and numerous techs that served as prerequisites for all that stuff (such as Antimatter Power, The Dark Side, and Artificial Satellites).
In fact, the space monsters were going to function similarly to escaped Galactic Prison convicts: a UP resolution would put a zoo on someone's planet, and then the monsters would break out and start wrecking havoc.
Like all that other stuff I mentioned, though, there were too many complications and not enough time to resolve them. I imagine they're planning on finishing up with those space monsters for the Dark Avatar expansion, though, like they are with terror stars.
