Zarrengibbit of the Caverns

Vigilance, trample
Solve the Riddle — At the beginning of each
opponent's upkeep, they may guess the name of
the card on the top of your library. If they do,
reveal the top card of your library. If the guess
was incorrect, Zarrengibbit deals 6 damage to
that player. If the guess was correct, Zarrengibbit
gains defender and loses all other keywords and abilities.