In Martian Fluxx, the players are invaders from Mars, and the Creepers are those pesky Pathetic Humans who are always getting in our way, keeping us from winning. Cows of course are Keepers because for Humans and Martians alike, they're good eating. There are actually two Cow Keepers in the game; if you have them both you can win with the Goal "Two All-Beef Earthlings."
The idea of making us the Martians came about as I realized the Martians are the ones who get all the cool stuff! Other cool Keepers you'll find include the Abduction Chamber (which you can hide your Pathetic Humans in), the Space Modulator and the Mind-Control Transmitter. And yes, you can use that Ray Gun to vaporize Pathetic Humans. (The Germs Creeper however is harder to get rid of...)
