River Merfolk

The mountainwalk activation is pure metagame insurance: a 2/1 body for that you cast on rate and then turn on, one mana at a time, only when the opponent happens to have shown up with red. Against anyone else the ability is a dead button, which is the whole trade. Wizards priced the creature as a fair beater and bolted on an upside that pays out against a slice of the field rather than all of it, the same conditional-evasion logic that gave the period its swampwalk and islandwalk creatures. The repeatable activation is the wrinkle that separates it from the static landwalkers: you are not locked into evasion at deploy time, so you can leave the merfolk back on defense and only commit mana on the turn you want to swing through. It is a tightly bounded design, more a snapshot of how the early game thought about color-hosed aggression than a card with a life beyond it; the landwalk axis it sits on faded as Magic moved toward evasion that did not depend on the opponent's exact mana base.