Primeval Shambler

The firebreathing template wearing black mana instead of red, and the swap tells you something about how the two colors were costed in the early era. Red's firebreathers (Shivan Dragon being the touchstone) tend to pair the pump with evasion, so the mana you sink turns into damage that lands. Black gets the same +1/+1-per-mana engine here on a ground body with no way through, which is why the math rarely pays: a 3/3 you can grow still has to connect, and a single chump blocker resets the investment to zero. The pump is at least instant-speed, so the mana sits open as a combat trick or a way to slip under a damage-based removal spell, and the activations are unbounded, meaning a late game flush with black sources converts directly into a finisher. But the Mercenary tag points at the real intent: this was built for the Mercenary tutoring chain, an engine that fetched smaller Mercenaries onto the battlefield, and a creature whose entire value is a mana sink fit a deck that wanted somewhere to dump its surplus lands. As a standalone card the rate is poor; as the top of a curve in a tribe designed to swarm and then run out of things to spend mana on, it had a job.