Gregor Clegane, The Mountain

Undying (When it dies, if it had no +1/+1 counters on it, return it to the battlefield under
its owner's control with a +1/+1 counter on it.)

Whenever Gregor Clegane, The Mountain, enters the battlefield, if it has a +1/+1
counter on it, it becomes a zombie in addition
to its other types.