Kobolds of Kher Keep

Costing nothing is the whole card. Stapling "free" to "is a creature" produces a body the rules treat as two orthogonal properties (mana spent to cast it versus the count of creatures you control), and combining them yields storm fuel, sacrifice fodder, convoke fodder, and a tribal anchor all at once from a single 0/1. The body was a curiosity in 1994; it became a problem the moment Prossh, Skyraider of Kher and Rohgahh of Kher Keep gave the Kobold tribe payoffs, and a bigger one once engines started caring about how many creatures or artifacts you control rather than how much mana you spent to deploy them. Wizards has spent the intervening decades being careful with free creatures: Memnite exists, Ornithopter exists, but the Kobolds have largely been left in their original printing and the handful of supplemental sets that acknowledged them. The lesson the card teaches is a pricing rule the modern game writes around rather than re-tests: if a creature costs nothing, every payoff that scales on creature count has to be costed as if this card is already in the format. That is why free creatures are a closed design space now, and the Kobolds are part of what closed it.