This question has also baffled me as I had guinea pigs as a kid and as you say they just do their business whenever and wherever. I have rescued a bunny rabbit 3 weeks ago and the only reason I found out he was litter trained was that he went straight into my cats litter tray and did his business. I was most impressed and part of that has made me decide to keep him but I've no idea how somebody managed to train him and I'll be interested to hear the replies you get !!!!