Vesuvan Doppelganger

The original cloning template, and the one every subsequent shapeshifter design has been measured against. Where Clone settles into a static copy and stays there, this one keeps the choice live: each upkeep, it can shed its current identity and assume a new one, carrying the upkeep clause forward so the loop never closes. That recursive self-reference (the copy ability copies itself) is the design idea, and it is genuinely strange for a card from the game's first print run, where templating was still being invented in real time. The exclusion of color is the other piece worth noticing: the Doppelganger keeps its blue identity no matter what it imitates, which mattered in an era when protection from a color was the dominant evasion and removal axis. Power and toughness, abilities, type line, everything else transfers; the color stays blue. Decades of clone variants have refined the rate and simplified the templating (Phantasmal Image, Phyrexian Metamorph, Spark Double), but none of them have brought back the shape-shifting-each-turn engine, because a copy effect that can re-target itself every upkeep is harder to balance than it looks. The reset is optional, an "at the beginning of your upkeep, you may" trigger rather than any kind of cost, which means the card can hold a shape indefinitely and only switch when a better target appears. What survives on the card is a record of what the original R&D thought a shapeshifter should be, before "copy" had settled into the clean transactional keyword it is now.