Is Mercurial Pretender legal in Standard, Modern, and Commander?

Mercurial Pretender is legal in: Modern, Pioneer, Legacy, Vintage, Commander.

What type of card is Mercurial Pretender?

Mercurial Pretender is a Creature — Shapeshifter. Its oracle text reads: "You may have this creature enter as a copy of a creature you control, except it has "{2}{U}{U}: Return this creature to its owner's hand." *The king went off to find himself. Imagine his terror when he succeeded.* Power/Toughness: 0/0"