Plague Mare — ruling 1

Plague Mare’s second ability affects only creatures your opponents control at the time it resolves. Creatures they begin to control later in the turn won’t get -1/-1.

Plague Mare : règles, fonctionnement et interactions (MTG)

Plague Mare 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

This creature can't be blocked by white creatures. When this creature enters, creatures your opponents control get -1/-1 until end of turn.

How it works : Plague Mare

Original English text — Wizards of the Coast / Scryfall

#1July 13, 2018· Wizards of the Coast
Plague Mare’s second ability affects only creatures your opponents control at the time it resolves. Creatures they begin to control later in the turn won’t get -1/-1.
No other card is named in this ruling.