Hackworth's answer doesn't cover the situation where a creature is being modified as it is entering the battlefield, nor the situation where another continuous effect with a more recent timestamp is modifying that layer. In that situation, Xenograft doesn't work.
Xenograft is a Static Ability with a Continuous Effect.
604.3a A static ability is a characteristic-defining ability if it meets the following criteria: (1) It defines an object’s colors, subtypes, power, or toughness;
603.6b Continuous effects that modify characteristics of a permanent do so the moment the permanent is on the battlefield (and not before then). The permanent is never on the battlefield with its unmodified characteristics. Continuous effects don't apply before the permanent is on the battlefield, however (see rule 603.6e).
Example: If an effect reads "All lands are creatures" and a land card is played, the effect makes the land card into a creature the moment it enters the battlefield, so it would trigger abilities that trigger when a creature enters the battlefield. Conversely, if an effect reads "All creatures lose all abilities" and a creature card with an enters-the-battlefield triggered ability enters the battlefield, that effect will cause it to lose its abilities the moment it enters the battlefield, so the enters-the-battlefield ability won't trigger.
Xenograft will not apply to is replacement effects that modify a creature AS it is entering the battlefield. From the 4th Ruling of Bramblewood Paragon:
4/1/2008 The creature gets the counter if its copiable characteristics as it would exist on the battlefield include the specified creature type. For example, say you control Conspiracy, and the chosen creature type is Warrior. If you put a creature onto the battlefield that isn't normally a Warrior, it won't get a counter from Bramblewood Paragon.
Additionally, Xenograft doesn't apply to (as the rulings say)
Creature cards not on the battlefield and creature spells are not affected.
Creature cards can exist in zones other than the battlefield (hand, exile, graveyard, out of the game, etc.), but you can only control creature (permanents) on the battlefield, not any other place. Xenograft only affects creatures you control, so it cannot apply to cards in other zones.
108.4. A card doesn’t have a controller unless that card represents a permanent or spell; in those cases, its controller is determined by the rules for permanents or spells. See rules 110.2 and 111.2.
Creature spells can only exist on the stack, not any other place.
111.1. A spell is a card on the stack.
Xenograft only affects creatures on the battlefield, so it cannot apply to spells.
Finally, if another continuous effect modified the same layer as Xenograft, it the ability that generated that continuous effect had a more recent timestamp, Xenograft's continuous effect would be overridden.
613.1d Layer 4: Type-changing effects are applied. These include effects that change an object’s card type, subtype, and/or supertype.
613.2. Within layers 1–6, apply effects from characteristic-defining abilities first (see rule 604.3), then all other effects in timestamp order (see rule 613.6). Note that dependency may alter the order in which effects are applied within a layer. (See rule 613.7.)