Infernal Pet — ruling 1

The triggered ability can trigger only once each turn. The ability will resolve before the second spell does. It doesn’t matter if the first spell you cast that turn has resolved, was countered, or is still on the stack.

Infernal Pet — ruling 2

The triggered ability triggers only if the creature that has it is on the battlefield as you cast your second spell. Spells you cast in a turn before that creature entered the battlefield will count. In other words, the ability won’t trigger if the creature with the ability is the second spell you cast during a turn or if you have already cast two or more spells by the time that creature enters the battlefield that turn.

Infernal Pet : règles, fonctionnement et interactions (MTG)

Infernal Pet 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

Whenever you cast your second spell each turn, put a +1/+1 counter on this creature and it gains flying until end of turn.

How it works : Infernal Pet

Original English text — Wizards of the Coast / Scryfall

#1February 5, 2021· Wizards of the Coast
The triggered ability can trigger only once each turn. The ability will resolve before the second spell does. It doesn’t matter if the first spell you cast that turn has resolved, was countered, or is still on the stack.
No other card is named in this ruling.
#2February 5, 2021· Wizards of the Coast
The triggered ability triggers only if the creature that has it is on the battlefield as you cast your second spell. Spells you cast in a turn before that creature entered the battlefield will count. In other words, the ability won’t trigger if the creature with the ability is the second spell you cast during a turn or if you have already cast two or more spells by the time that creature enters the battlefield that turn.
No other card is named in this ruling.