Dunerider Outlaw — ruling 1

Dunerider Outlaw's triggered ability gives it at most one +1/+1 counter each turn, regardless of how many opponents were dealt damage beyond the first.

Dunerider Outlaw — ruling 2

If Dunerider Outlaw dealt damage to an opponent who later left the game before that turn's end step, its ability still gives it a +1/+1 counter.

Dunerider Outlaw — ruling 3

Dunerider Outlaw's triggered ability checks to see if it dealt damage during the turn to an opponent of its current controller. If it dealt damage to player A, then player A takes control of it, the ability doesn't trigger.

Dunerider Outlaw : règles, fonctionnement et interactions (MTG)

Dunerider Outlaw 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

Protection from green At the beginning of each end step, if this creature dealt damage to an opponent this turn, put a +1/+1 counter on it.

How it works : Dunerider Outlaw

Original English text — Wizards of the Coast / Scryfall

#1March 19, 2021· Wizards of the Coast
Dunerider Outlaw's triggered ability gives it at most one +1/+1 counter each turn, regardless of how many opponents were dealt damage beyond the first.
No other card is named in this ruling.
#2March 19, 2021· Wizards of the Coast
If Dunerider Outlaw dealt damage to an opponent who later left the game before that turn's end step, its ability still gives it a +1/+1 counter.
No other card is named in this ruling.
#3March 19, 2021· Wizards of the Coast
Dunerider Outlaw's triggered ability checks to see if it dealt damage during the turn to an opponent of its current controller. If it dealt damage to player A, then player A takes control of it, the ability doesn't trigger.
No other card is named in this ruling.