Oracle Text

As Xenograft enters the battlefield, choose a creature type.

Each creature you control is the chosen type in addition to its other types.

Card Rulings

6/1/2011 You must choose an existing _Magic_ creature type.
6/1/2011 Creature cards not on the battlefield and creature spells are not affected.
9/29/2017 Replacement effects that modify creatures of a certain type as they enter the battlefield will apply (or not apply) after you apply this effect. For example, if Warrior is the chosen creature type and you control Bramblewood Paragon, a Runeclaw Bear would enter the battlefield with an additional +1/+1 counter.