The Red Terror

Growth that scales with your whole board's aggression, not just this creature's own combat. The Advanced Species trigger fires on any red source dealing damage to permanents or players, which means every burn spell, every attacker, every ping, and every direct-damage instant you cast feeds a single counter onto a body that arrives at 4/3. The design ties a legendary creature's size to the density of your red damage output rather than to protecting or blinking the creature itself, so the incentive runs toward spreading damage wide: a swarm of small attackers, a suite of cheap burn, or a source that hits multiple targets at once (which still resolves as a single trigger, since the ability cares about the event, not the number of things damaged). The wrinkle worth noting is that the counter lands after the damage, so it does not retroactively enlarge whatever dealt that damage; it compounds turn over turn, growing this creature into a threat that the rest of your red spells built for you. It is a payoff piece that asks you to keep the damage flowing rather than a self-contained engine, and its ceiling is a function of how relentlessly the deck around it points red damage in every direction.