Waterwalker Stag

If Waterwalker Stag was in your opening hand at the beginning of the game, you may reveal it. If you do, you may cast it without paying its mana cost.
When Waterwalker Stag enters the battlefield, memorize green. (The next time any player casts a green spell before your next turn, if the amount of mana spent to cast it is less than the amount spent to cast Waterwalker Stag, create one copy of that spell without memorize. You may choose new targets for the copy.)