Mutilate — ruling 1

The effect is based on the number of Swamps you control when Mutilate resolves. Only creatures on the battlefield at that time will be affected. The effect won't change later in the turn, even if the number of Swamps you control changes.

Mutilate : règles, fonctionnement et interactions (MTG)

Mutilate 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

All creatures get -1/-1 until end of turn for each Swamp you control.

How it works : Mutilate

Original English text — Wizards of the Coast / Scryfall

#1July 1, 2012· Wizards of the Coast
The effect is based on the number of Swamps you control when Mutilate resolves. Only creatures on the battlefield at that time will be affected. The effect won't change later in the turn, even if the number of Swamps you control changes.
No other card is named in this ruling.