Is Hidden Dragonslayer legal in Standard, Modern, and Commander?

Hidden Dragonslayer is legal in: Modern, Pioneer, Legacy, Vintage, Commander.

What type of card is Hidden Dragonslayer?

Hidden Dragonslayer is a Creature — Human Warrior. Its oracle text reads: "Lifelink Megamorph {2}{W} (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its megamorph cost and put a +1/+1 counter on it.) When this creature is turned face up, destroy target creature with power 4 or greater an opponent controls."