Zhur-Taa Ancient

The doubling effect here is symmetric, and that is the whole tension: every land tap, yours or your opponent's, returns an extra mana of the same type. The big body is the disguise. A 7/5 for five reads like a beatdown payoff, but the trample-less, evasion-less stat line is a deliberate brake on a card whose text is pure acceleration. Mana Flare and Heartbeat of Spring did the same symmetric-ramp work as enchantments, fragile and bodiless; folding that effect onto a creature changes the calculus entirely, because the ramp now also has to be killed, and killing a 7/5 is not free. The Gruul color pairing matters too: red and green are the colors least equipped to spend a sudden mana surplus on card advantage, so the doubling tends to favor whoever has the most expensive thing to cast, which is usually the deck running a five-mana 7/5 in the first place. What it does not do is fix anything; it adds a mana of a type the land already produced, so a Mountain still only makes red, just twice as much. The effect rewards a board built before the Ancient arrives rather than one assembled after, and the symmetry means it punishes the durdling opponent far more than the player who deployed it with a plan to immediately convert the extra mana into pressure.