Aerie Bowmasters
Creature — Dog Archer
Reach (This creature can block creatures with flying.)
Megamorph 
(You may cast this card face down as a 2/2 creature for 
. Turn it face up any time for its megamorph cost and put a +1/+1 counter on it.)
Matt Stewart
Dragons of Tarkir (DTK)
• #170Rulings
Turning a face-down creature with megamorph face up and putting a +1/+1 counter on it is a special action. It doesn't use the stack and can't be responded to.
(wotc • 2015-02-25)
Megamorph is a variant of the morph ability. You can find more information on morph on cards with morph from the Khans of Tarkir set.
(wotc • 2015-02-25)
If a face-down creature with megamorph is turned face up some other way (for example, if you manifest a card with megamorph and then pay its mana cost to turn it face up), you won't put a +1/+1 counter on it.
(wotc • 2015-02-25)
Rulings
Turning a face-down creature with megamorph face up and putting a +1/+1 counter on it is a special action. It doesn't use the stack and can't be responded to.
(wotc • 2015-02-25)
Megamorph is a variant of the morph ability. You can find more information on morph on cards with morph from the Khans of Tarkir set.
(wotc • 2015-02-25)
If a face-down creature with megamorph is turned face up some other way (for example, if you manifest a card with megamorph and then pay its mana cost to turn it face up), you won't put a +1/+1 counter on it.
(wotc • 2015-02-25)