Cryptic Trilobite

A mana battery that pays you back in a specific currency, and the restriction on that currency is what makes it interesting rather than broken. The counters do double duty here: they set the body's size at cast time and act as a reservoir you crack for colorless mana, but that mana is fenced off, spendable only on abilities. So the loop it wants is not ramp toward a big spell; it is fuel for activated-ability engines, the kind of mana sink that eats colorless in bulk. Sink the counters into an activation, then rebuild them one at a time with the tap ability, and the creature becomes a slow, self-replenishing generator that also happens to grow into a threat if left alone. The design tension is between those two roles: every counter you spend for mana shrinks the body and delays the point where it fights, and every turn spent tapping to reload is a turn it does not attack. That the tap ability costs mana to add a counter, while removing one gives you double, means the arbitrage only works if you have somewhere to dump colorless faster than you can rebuild. It rewards decks built around expensive activated costs (equip fees, level-up sinks, monstrosity, big fireball-style X abilities that key off colorless) rather than decks that just want a fat blocker.