Rootwalla

Pump-on-a-stick was old news in green long before this lizard arrived: Killer Bees and Shivan Dragon had been spending mana for stats for years. What this did was miniaturize the idea and discipline it. The firebreathing-style activation parks excess mana into a threat that demands an answer before it can swing big, then the body shrinks back to a 2/2 the rest of the time. The "only once each turn" clause is the friction doing all the balancing work; without it, another green and another colorless would buy another +2/+2, and another, until the card stopped being a creature and started being an arbitrarily large threat off a single open turn. That one line caps the body at 4/4 in any given combat and separates a clean curve-filler from an engine, and the lever (capped firebreathing on a small green body) has shown up on aggro creatures ever since. It also opened a naming lineage: Basking Rootwalla later took the same scaling lizard and rebuilt it for graveyard-recursion shells, but the template starts here, with a body that asks you to spend your last two mana profitably instead of holding up nothing. The right cost, a hard ceiling on the activation, and a pump clause restrained at exactly the point where restraint becomes elegance.