Phyrexian Hydra

Most infect creatures want to live: poison counters only matter if the body survives long enough to connect. This one inverts the contract. The damage-prevention clause looks like protection, but it converts every point of incoming damage into a -1/-1 counter on its own body, which means a 7/7 that blocks or trades is steadily eating itself from the inside. A single block against a modest attacker shaves it down; chump it twice and the prevention has done the opponent's killing for them. The result is a body built for one direction only: forward, unblocked, ideally on the turn it can end things. Poison's brutal math makes that worth chasing, since a 7-power infect attacker is more than half a poison kill in a single swing, but the self-counter mechanic forecloses the patient game entirely. You cannot sit back behind it, cannot trade it into the red zone, cannot use it as a wall. It is an all-gas-no-brakes design wearing the costume of a sturdy green fatty, and the prevention clause is the catch that keeps the rate honest: the toughness is real until anything points damage at it, at which point the toughness pays the bill. The interaction with -1/-1 counters as a damage-handling currency also makes it a poor candidate for combat math that assumes traditional damage marking, since nothing here marks; it withers instead.