Centaur Chieftain — ruling 1

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.

Centaur Chieftain — ruling 2

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.

Centaur Chieftain : règles, fonctionnement et interactions (MTG)

Centaur Chieftain is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Haste Threshold — As long as there are seven or more cards in your graveyard, this creature has "When this creature enters, creatures you control get +1/+1 and gain trample until end of turn."

How it works : Centaur Chieftain

Original English text — Wizards of the Coast / Scryfall

#1June 8, 2016· Wizards of the Coast
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.
No other card is named in this ruling.
#2June 8, 2016· Wizards of the Coast
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.
No other card is named in this ruling.