Check out this trailer for Aliens Vs. Predator's Survivor Mode, in which up to four players co-operatively take on the Alien hordes.

I suppose it's time to set aside the comparisons to Gears of War 2 and simply accept Horde Mode as a standard game type in online multiplayer shooters. It's just the natural progression of things. Remember back in 1990 when the term for first-person shooter was "Doom clone?" We've moved on from that, now we can move on from this.