I was riding a bus from my house in the Scottish Borders up to Edinburgh once and was reflecting on the idea during the ride. That idea (growable tree house) came to me after giving considerable thought to a new mode of transportion- an animal that was engineered genetically to be dependent upon a computer CPU for its brain. You could combine all the best features of antelope, horse, cheetah, kangaroo etc and design the rib cage of the creature to form a sort of cockpit for you to nestle down in. The animal would still have a vestigal brain perhaps- a little node at the top of the spinal cord that did the little tedious things that keep an organism going (crapping, heart rate that sort of thing) while the computer would take over higher functions like vision, course determination, specific commands (like "Speed up!" "Stop" "Trample that Democrat") etc. You could design the creature to break down just about any plant matter- grass, leaves, fruit whatever- just open up a port on the creature's side and shovel the stuff in...
It would keep you warm if you were cold, respond to verbal commands and be fueled by whatever you could find along the way. You could perhaps even add a compressor to the rear end of the varmint and compress all that methane produced, bottling it for later use as a fuel source at home...