Polyraptor

Enrage usually reads as a defensive incentive: deal me damage and I punish you. Here the punishment is recursion run amok, because the token the trigger makes is a copy of the same creature, which means it carries the same trigger. Damage one Polyraptor and you get two; deal damage to both and you have four; the count doubles every time a damage source touches every copy at once. The self-sustaining part is that the trigger fires whenever the creature is dealt damage, lethal or not, and the token is created regardless of whether the original survives the hit. Deal five to one of these: state-based actions are checked first, so the original dies to the lethal damage, and only then does the enrage trigger resolve into a fresh copy. The body that took the killing blow is gone, but the population held steady. That makes a source dealing a fixed, non-lethal point of damage repeatedly (one that never outright kills the 5/5) an arbitrarily large army generator, since each hit adds a body and none subtract one; a symmetrical damage source (a sweeper that undershoots, a fight effect, a damage-based loop) hits every copy simultaneously and turns the whole board into a chain reaction. The discipline keeping it out of pure-combo territory is the cost: eight mana for a 5/5 walls the doubling off from the early game, where assembling it would be cheap and fast. This is enrage taken to its self-replicating extreme, where the drawback and the payoff are the same clause.