Plaguemaw Beast

Proliferate as a repeatable engine rather than a one-shot spell, and the price for repetition is paid in creature bodies. Most early carriers of the keyword stapled it to a single moment: an enters-the-battlefield effect, a cast trigger, a spell that fired once and was gone. Here the keyword sits behind a tap-and-sacrifice activation, which turns it into a recurring tax you levy on your own board: feed it a creature each turn and every counter you care about advances another step. That sacrifice clause is what keeps the rate from running away. Proliferate touches everything at once (the +1/+1 counters on your team, the loyalty on your planeswalkers, charge counters on artifacts, poison on your opponent), so a free, untapped version would be absurd; demanding a body per activation forces you to build a board worth converting into counters rather than spinning the engine for nothing. The synergy it wants becomes obvious once you read the activation cost as an asset: token generators and creatures that already plan to die make the sacrifice nearly free, so each death you were going to take anyway also nets a proliferate. As a Phyrexian Beast, the flavor lines up with the corruption it advances, poison counters in particular spreading the way the creature itself feeds on flesh to grow. The 4/3 body is incidental; what you are buying is a counter-multiplier you crank once per turn, for as long as you have fuel.