Drifters Coin

{b}{b}{b}, Sacrifice Drifters Coin: Choose target
creature you control, then flip a coin. If you win
the flip, double the power and toughness of the
chosen creature until end of turn, if you lose the
flip that creature becomes a 1/1 green snake
creature until end of turn.
"You lookin' to do business?"
—The Drifter