Feral Hydra

The pump ability belongs to everyone, and that is the whole design conceit. Granting "any player may activate this ability" turns a private growth engine into a shared lever: an opponent can feed counters onto this creature, and the real question is why they ever would. The answer lives in the same counter math that makes it grow. A wide-open board, a politically tense table, an opponent who wants to inflate the body just enough to bait a removal spell or to push it out of range of a damage-based answer: the open activation manufactures negotiations and traps that a closed pump ability never could. The X in the casting cost sets the entry size and the open ability sets the ceiling, so the creature scales twice, once on announcement and indefinitely thereafter, for anyone with three mana to spare. The catch is that three generic mana per counter is a steep rate to grow it alone, which keeps the body honest and pushes the card toward the table dynamics it was built to provoke rather than toward raw efficiency. It is a green fattie wearing a social mechanic, the kind of open-activation experiment that surfaces rarely because it hands the sizing decision to the table rather than to the controller, and trusts the threat of removal to police whatever the table agrees to build.