All creatures under your control when Centaur Chieftain’s triggered ability resolves are affected, including Centaur Chieftain itself. Ones that come under your control or become creatures later in the turn are not.

If you don’t have enough cards in your graveyard at the moment Centaur Chieftain enters the battlefield, its threshold ability won’t trigger. If the number of cards in your graveyard changes and Centaur Chieftain loses the ability once it has triggered, the ability will still resolve.