Placid Rottentail

The design trick is that the body and the payoff are billed to different turns and different resources. On the battlefield it is a vigilant one-drop, small enough to trade and attack without dropping your guard. Once it dies, it stops being a creature and becomes a graveyard reservoir: a delayed pump that exiles itself to strap two counters onto whatever is still standing. You pay one mana up front for a warm body, then pay again later for the counters, and the sorcery-speed restriction on the graveyard activation keeps it from doubling as a combat trick or a surprise blocker booster. It rewards a board that wants to keep growing after the first wave has been traded off, which is exactly the shape of a counters-matters deck: the Rottentail dies into the later game rather than out of it. The Fungus Rabbit typing is the flavor gag (a toadstool sprouting from a rabbit that keeps swinging), but the counters clause is the real work, converting a spent one-drop into a permanent statline upgrade for a creature you actually intend to win with.