Plaxmanta — ruling 1

If this enters in a way other than announcing it as a spell, then the appropriate mana can't have been paid, and you'll have to sacrifice it.

Plaxmanta : règles, fonctionnement et interactions (MTG)

Plaxmanta 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

Flash When this creature enters, creatures you control gain shroud until end of turn. (They can't be the targets of spells or abilities.) When this creature enters, sacrifice it unless was spent to cast it.

How it works : Plaxmanta

Original English text — Wizards of the Coast / Scryfall

#1May 1, 2006· Wizards of the Coast
If this enters in a way other than announcing it as a spell, then the appropriate mana can't have been paid, and you'll have to sacrifice it.
No other card is named in this ruling.