Balduvian Hydra

The +1/+0 counter doing two jobs at once is the entire engine here, and it dates to a time when designers were still probing how much weight a single counter type could carry. You pay X to load the body up, and every counter both swings power and stands ready as a one-shot prevention shield. Those mandates fight each other directly: each point of damage you soak costs a permanent point of power, so the creature whittles itself down to stay alive. The prevention is an activated ability with no once-per-turn cap, though, so the rationing is the player's choice rather than the rules': feed three counters into the shield at instant speed and you blunt a burn spell or a big attacker outright, but you have just spent three points of power to do it. The regrowth clause rebuilds the stockpile, yet it fires only on your upkeep and at three red mana per counter, so it competes with everything else you would rather spend that turn on. The result is a finite pool of counters to ration across attacking and defending, every defensive activation a permanent tax on the offense. It is fiddly and math-heavy, and the deliberate decision to make prevention a counter-for-damage trade keeps the apparatus from ever snowballing. Few players reach for it, but it is a genuine fossil of counters serving as both stat-line and soak, well before that flexibility became a standard tool.