Horseshoe Crab

Untapping yourself for one blue mana is a deceptively load-bearing line of text. By itself, the crab is a defensive 1/3 that can swing for one and immediately stand back up to block, which is to say it does almost nothing. The point is what you bolt onto it. Strap on an Aura or Equipment that grants a tap ability (anything that produces mana, draws a card, or fires a trigger when the creature taps), and the clause becomes a loop bounded only by how much blue mana you can muster. The crab supplies the untap; the attachment supplies the payoff; together they recur as many times as your mana allows. That open-endedness is exactly why the effect rides such a weak body. A 1/3 that contributes nothing to combat means the untap clause never has to be priced against an aggressive stat line, so the cost can stay at a flat single mana and scale with your infrastructure rather than with the card itself. Magic has printed plenty of self-untapping creatures, but the crab is the leanest expression of the template: a minimal frame whose entire value is the repeatable untap, inert until you supply the second half, then unbounded once you do.