Phyrexian Gremlins

The "stay tapped to keep a thing locked down" template, and a near-prototype of it. The design trick is elegant: by letting you decline to untap the gremlin itself, the card converts a one-shot tap effect into an indefinite lock, paying for it with the creature's own availability. Tap an opposing artifact, leave the gremlin tapped, and that artifact stays off until you free the gremlin by untapping it. The cost is that you have surrendered a body and a tap-permission you might want elsewhere. That trade reads as fair on paper and was probably overcosted in practice, but the mechanical idea (a creature whose tapped state is the resource, not its combat) has shown up in cleaner forms ever since, from the various "doesn't untap" auras and creatures that followed to designs that lock a permanent down by anchoring their own. The artifact-only targeting is era-appropriate: this was printed into the set where artifacts were the problem to be solved, and black's answer was to grab one by the throat and hold on. A historical curio rather than a card with a format career, but worth reading as a prototype: the lock-by-staying-tapped clause is one of the small design vocabularies Magic has kept refining for decades, and this is close to where it starts.