Gating

Gating is a term that refers to cards that require you to return a creature that shares a color with it to your hand when it comes into play in exchange for a lower casting cost. Gating cards were introduced in Planeshift. Gating is not an ability or a keyword.

List of Gating cards:

Cavern Harpy Doomsday Specter Flletfoot Panther Horned Kavu Lava Zombie Marsh Crocodile Natural Emergence

Razing Snidd Sawtooth Loon Shivan Wurm Silver Drake Sparkcaster Steel Leaf Paladin

The most powerful of these is the Shivan Wurm, a 7/7 trampler which could be cast third turn in a green deck with mana-producing creatures.