Belt of Giant Strength

The joke is baked into the equip cost. Ten generic mana to strap a creature into a flat 10/10 body is a rate nobody would pay, until you read the second clause: the cost drops by the power of the target. Point it at a big enough attacker and the equip becomes free, which inverts the usual Equipment logic. Most equipment rewards you for suiting up a small creature and turning it into a threat; this one asks you to already have a threat, then flattens it to 10/10. That makes it a lightning rod for the corner cases where a creature's power is arbitrarily large: anything that pumps into the double or triple digits equips this for zero, and the base 10/10 becomes a floor rather than a ceiling when combined with further static buffs. It is also a quiet answer to a certain kind of vulnerability, since setting base power and toughness overwrites the small stat lines that other effects have shrunk a creature down to. The design lives entirely in that scaling clause: a number that looks unpayable on its face, engineered to be trivially payable in exactly the decks that want a 10/10 walking around for two mana of setup.