Lightning Serpent — ruling 1

If Lightning Serpent enters without X being paid because an effect puts it onto the battlefield or an effect lets you cast it without paying its mana cost, then X is 0.

Lightning Serpent : règles, fonctionnement et interactions (MTG)

Lightning Serpent 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

Trample, haste This creature enters with X +1/+0 counters on it. At the beginning of the end step, sacrifice this creature.

How it works : Lightning Serpent

Original English text — Wizards of the Coast / Scryfall

#1July 15, 2006· Wizards of the Coast
If Lightning Serpent enters without X being paid because an effect puts it onto the battlefield or an effect lets you cast it without paying its mana cost, then X is 0.
No other card is named in this ruling.