Marath, Will of the Wild

You can't choose to pay extra mana to cast a creature spell unless something instructs you to.

You announce the value of X as you activate the ability, and all instances of X in the activation cost are equal to the announced value. For example, if you choose 2 as the value of X, then you pay 2 and remove two +1/+1 counters to pay the cost.

If Marath enters the battlefield without being cast, then no mana was spent to cast it. It will therefore enter the battlefield without any +1/+1 counters. If no other effects are increasing its toughness at that time, it will subsequently be put into its owner's graveyard as a state-based action.