Midnight Guard

The untap clause does nothing for the body itself: a 2/3 standing back up is just a slightly resilient blocker until you give it something worth untapping for. The mechanism hiding inside is the second half of a free tap engine. Enchant Midnight Guard with an Aura whose tap ability costs no mana, and every other creature that enters resets the Guard so it can fire again. The canonical pairing is Presence of Gond: tap the Guard for a 1/1 Saproling, that Saproling entering untaps the Guard, tap it for another Saproling, and the board fills as fast as you can resolve triggers. The Guard is one cog, not the machine: it has no value in a vacuum, and its untap clause only ever points at itself, which is precisely why it sits idle in fair decks and lights up in combo shells built around a repeatable no-mana tap activation. That self-referential loop is the design's central problem. The Guard cannot break itself open; it needs an outside permission slip (the Aura) to convert "another creature entered" into "tap for the free thing again," and the moment that permission arrives, a nearly vanilla three-drop becomes an arbitrarily large token faucet. It is a build-around in the strictest sense: worthless to most, decisive to the one deck assembled around the right no-mana tap symbol.