What is the key difference between a multiparty government and the US government?

The US political system has basically reduced to just "Elephants & Donkeys" who have kicked every other "animal" out of their "playpen" by making it too expensive to enter and been interbreeding so much that they are in the process of becoming "Elekeys & Donkphants". Soon nobody will be able to tell them apart (except they will always be calling each other names and distributing attack advertising before elections, while refusing to make anything clear about themselves).

A multiparty political system would have whatever "animals" (e.g. "snakes", "horses", "cows", "dogs", "armadillos", "fish", "turtles", "frogs", "groundhogs", "cats", "yes, even the green things, even though nobody really knows what the green things are") that wanted to participate in the system.