Lone Rider // It That Rides as One — ruling 1

Lone Rider's last ability triggers even if it wasn't on the battlefield when you gained the life this turn.

Lone Rider // It That Rides as One — ruling 2

Lone Rider's ability checks how much life you've gained during the turn, not what your life total is compared to what it was when the turn began. For example, if you gained 4 life and then lost 7 life this turn, it still triggers.

Lone Rider // It That Rides as One — ruling 3

You must have gained life before your end step begins. Lone Rider's last ability won't trigger if you haven't, even if an ability triggering at the same time will cause you to gain life.

Lone Rider // It That Rides as One — ruling 4

In a Two-Headed Giant game, events that cause a player to gain life affect each player separately, even though the result affects the team's life total. For example, if your teammate's creatures with lifelink dealt 5 damage, your teammate gained life, not you, and Lone Rider's last ability won't trigger.

Lone Rider // It That Rides as One : règles, fonctionnement et interactions (MTG)

Lone Rider // It That Rides as One 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

First strike, lifelink At the beginning of the end step, if you gained 3 or more life this turn, transform this creature. First strike, trample, lifelink

How it works : Lone Rider // It That Rides as One

Original English text — Wizards of the Coast / Scryfall

#1July 13, 2016· Wizards of the Coast
Lone Rider's last ability triggers even if it wasn't on the battlefield when you gained the life this turn.
No other card is named in this ruling.
#2July 13, 2016· Wizards of the Coast
Lone Rider's ability checks how much life you've gained during the turn, not what your life total is compared to what it was when the turn began. For example, if you gained 4 life and then lost 7 life this turn, it still triggers.
No other card is named in this ruling.
#3July 13, 2016· Wizards of the Coast
You must have gained life before your end step begins. Lone Rider's last ability won't trigger if you haven't, even if an ability triggering at the same time will cause you to gain life.
No other card is named in this ruling.
#4July 13, 2016· Wizards of the Coast
In a Two-Headed Giant game, events that cause a player to gain life affect each player separately, even though the result affects the team's life total. For example, if your teammate's creatures with lifelink dealt 5 damage, your teammate gained life, not you, and Lone Rider's last ability won't trigger.
No other card is named in this ruling.