Hidden Stag

The flip between enchantment and creature, and who drives it, runs against the controller in a genuinely unintuitive way. An opponent's land drop turns this into a 3/2 Elk Beast, so it animates on their turn: available to block their attackers, but fully exposed to whatever sorcery-speed removal they cast during their own main phase. Your own next land flips it back to an inert enchantment, which means the on/off switch lives in the land sequence rather than in your hands. You can still swing on offense by attacking in combat and then making your land drop in the post-combat main phase, leaving the Stag animated through your own combat step before you reset it. The "if this permanent is a creature" qualifier is the lever the whole design turns on: a 3/2 for two would be undercosted in green, so the card surrenders state control to the routine act of playing lands, each player nudging the machine on their own turn. The trigger turns something both players do every turn into a shared switch neither fully owns, drawing on a land-matters tension common to early-era permanents. The practical result is a beater most vulnerable precisely on the turn it is a creature, sitting under enemy removal during their main phase: a genuinely strange product of a time still probing what an enchantment could be, more clever than competitive, and one that reads better than it plays.