Clone

The original copy effect, and the template that fixed what every shapeshifter since has had to clear. The design is almost startlingly clean: a 0/0 body that is meaningless on its own, paired with a replacement effect that resolves before the creature ever exists as a 0/0, so the stat line is a formality rather than a vulnerability. That trick (printing a creature whose printed stats are irrelevant because it rarely enters with them) is the entire conceit of the Shapeshifter type, and it starts here. Everything the color pie has done with copying since has been a variation on this chassis: Phantasmal Image cut the cost by adding a fragility clause, Phyrexian Metamorph widened the targets to artifacts and shifted into colorless, Sakashima of a Thousand Faces dodged the legend rule, Spark Double added a +1/+1 counter and a planeswalker mode. Every one is a designer tightening or loosening a single variable on the Clone equation. The rate has held at four mana for three decades because the effect scales with the board: a Clone is exactly as good as the best creature on the battlefield, a self-balancing power level that needs no further restriction. The lasting lesson here is that copying, as a mechanic, never needed a keyword to work; it needed a clear replacement-effect template, and this is where that template was written.