Adarkar Unicorn

A mana ability bolted directly to its set's signature tax mechanic, built to solve a problem its own block created. Cumulative upkeep stacked a growing toll on permanents every turn, and that cost was frequently colored: blue, especially, for the snow-water spells and the heavier enchantments. This 2/2 exists to feed that meter and nothing else. The mana it produces is fenced off, usable only against cumulative upkeep, which is why it sits in white rather than blue. It lets a white-based deck keep a blue-taxed permanent alive without committing to the second color's full manabase. That fencing is the entire design discipline. A general blue source on a white body would have been a quiet fixing upgrade with broad value; restricting the output to upkeep payments turns the body into a dedicated support piece that does one job and cannot be repurposed. This is what answers looked like when Magic still designed them narrow enough to be legible only against the specific rule they targeted, before keyword counters and modal flexibility made single-purpose creatures look quaint. The Unicorn is what a build-around looks like when the thing you are building around is a mechanic everyone hated and the card exists to make that mechanic survivable.