Fight or Flight — ruling 1

Creatures which enter (or cards which become creatures) after the beginning of combat ability resolves are not placed in either pile. Since they weren't in the chosen pile, they won't be able to attack.

Fight or Flight — ruling 2

If you have two of these, then you will separate the creatures into piles twice, and the opponent will choose a pile each time. Only creatures that were in both of the chosen piles can attack.

Fight or Flight — ruling 3

The effect is best understood if you read it as "creatures not in the chosen pile can't attack."

Fight or Flight : règles, fonctionnement et interactions (MTG)

Fight or Flight 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

At the beginning of combat on each opponent's turn, separate all creatures that player controls into two piles. Only creatures in the pile of their choice can attack this turn.

How it works : Fight or Flight

Original English text — Wizards of the Coast / Scryfall

#1August 1, 2008· Wizards of the Coast
Creatures which enter (or cards which become creatures) after the beginning of combat ability resolves are not placed in either pile. Since they weren't in the chosen pile, they won't be able to attack.
No other card is named in this ruling.
#2August 1, 2008· Wizards of the Coast
If you have two of these, then you will separate the creatures into piles twice, and the opponent will choose a pile each time. Only creatures that were in both of the chosen piles can attack.
No other card is named in this ruling.
#3August 1, 2008· Wizards of the Coast
The effect is best understood if you read it as "creatures not in the chosen pile can't attack."
No other card is named in this ruling.