Ruthless Deathfang — ruling 1

Ruthless Deathfang's triggered ability will trigger whenever you sacrifice any creature, including Ruthless Deathfang itself. It will trigger whenever you sacrifice a creature for any reason, including to pay a cost or as the result of a spell or ability (such as an opposing Ruthless Deathfang's ability).

Ruthless Deathfang — ruling 2

Notably, the “legend rule” does not cause any permanents to be sacrificed. They are simply put into their owners' graveyards.

Ruthless Deathfang : règles, fonctionnement et interactions (MTG)

Ruthless Deathfang 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

Flying Whenever you sacrifice a creature, target opponent sacrifices a creature of their choice.

How it works : Ruthless Deathfang

Original English text — Wizards of the Coast / Scryfall

#1February 25, 2015· Wizards of the Coast
Ruthless Deathfang's triggered ability will trigger whenever you sacrifice any creature, including Ruthless Deathfang itself. It will trigger whenever you sacrifice a creature for any reason, including to pay a cost or as the result of a spell or ability (such as an opposing Ruthless Deathfang's ability).
No other card is named in this ruling.
#2February 25, 2015· Wizards of the Coast
Notably, the “legend rule” does not cause any permanents to be sacrificed. They are simply put into their owners' graveyards.
No other card is named in this ruling.