Gogo, Master of Mimicry

Most copy effects target spells; this one targets the layer beneath them. Activated and triggered abilities are the currency of engine decks, and the older tools for doubling them were narrow: a Rings of Brighthearth here, an Illusionist's Bracers there, each locked to a single copy of a single ability. This tap ability is variable and repeatable. Because X can't be zero and pays twice into the copy count, the number of copies scales: point it at an enters-the-battlefield trigger, a planeswalker loyalty activation, a life-drain, or an ability that fetches more lands, and the ceiling is bounded only by how much mana you're willing to sink. The self-protection clause ("this ability can't be copied") is the tell that the designers saw the obvious loop coming and closed it: you cannot aim the copy engine at itself to go infinite for free. What keeps the design coherent rather than sprawling is the target restriction to abilities you control, which anchors it to your own board rather than the whole table. And because the ability carries no timing restriction, it works on the stack: let an opponent respond to your original ability, then multiply it at instant speed in reply. The 2/4 body is durable enough to survive a turn, which matters for a permanent whose whole value is surviving to be tapped. The result is less a copy card than a multiplier bolted onto whatever ability the rest of the board is already producing.