The controller of the creature chooses which one
Untapping and re-activating artifacts is easy as pie in EDH, with enough mana rocks and a Dramatic Reversal stapled onto an Isochron Scepter (plus all seven Bobbleheads), we can just roll the dice 50,000 times until we roll seven 6s
This is an ability that shines brightest when its on an already decent card that gets better when your hand is empty
Actually, there's no ethical or acceptable reason to make "proxies" to pass the light test, bend test, and other well-known tests to weed out fake cards because there's no ethical or acceptable use of proxies that requires the proxy to be as similar to a real card as possiblea Sharpie on a basic Island does the job just as well and without any of the legal and moral issues
Totentanz, Swarm Piper Totentanz, Swarm Piper was a signpost uncommon of the Wilds of Eldraine Limited format, making use of rats and creatures that make Rat tokens when they die