To turn into a Reaper, achieve the highest score on a winning team. To turn into a Giant Chicken, achieve the lowest score on the losing team. Both have unlimited life. The chicken can lay rainbow eggs that will turn another person into a reaper.
Want to be the Reaper or Giant Chicken but weren't at the top of the heap? Neither the Chicken nor Reaper has infinite life; they're just tough to kill. If you can kill them (they have approximately 16-20 hearts), when they die they will produce several rainbow eggs. Pick up one of these orbs like a hat to become a Reaper or Giant Chicken-- depending on which one you killed. About 6-8 orbs drop from one Reaper, so a couple people can become the Reaper at the same time.
Choose the "Play With Yourself" option at the main menu and select "Mess About". Pick any desired type of game, then start the game and press Start to pause. Finally, press UP(2), DOWN(2), LEFT, RIGHT, LEFT, RIGHT, SELECT, START, CIRCLE