Whenever you sacrifice a creature, target opponent sacrifices a creature.
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).
Notably, the “legend rule” does not cause any permanents to be sacrificed. They are simply put into their owners’ graveyards.