Z-rex

Z-rex enters the battlefield tapped and doesn't untap during your untap steps. At any time you may sacrifice a creature you control to untap Z-rex.
{t}{2}{b}{b}, sacrifice a non-zombie creature and put X +1/+1 counters were X equals sacrificed creatures toughness.
"Now if we could just keep him from eating our guys" -Tiber'zosh, Necromancer