Cancer is a mutated unicorn which, after being left behind on Noah's Ark, went to join the Dinosaur race underneath the crust of the earth. It was brought out into the surface as a result of the nuclear bomb striking and causing the Dinos' to become extinct again (which is sensible when their only weakness is large, unidentified objects flying from the sky and nearly destroying the Earth in it's entirety). The lone unicorn suffered radiation poisoning after eating the Dino's bodies for lack of food, and somehow mutated into the Cancerous Monster you see before you. It feeds upon rainbows and the energy of Chuck Norris (noun). Therefore it is nigh invincible, and your best plan of action is to RUN BYOTCH, RUN!