Zelda II: The Adventure of Link
January 14, 1987
In the Japanese version of the game, in the Water Town of Saria, there's a grave that reads: "The hero, Loto, rests here." This references Loto (ロト) an ancient legendary hero from Dragon Quest.
Some believe this to imply an In-universe link between the two series, considering the hero from Dragon Quest to be buried in the Zelda universe, but since this is not a licensed cameo, we can't consider that hero the same as the one from Dragon Quest.
This reference, however, was removed from international releases of the game.
The creators of Zelda II held no right over Enix's Dragon Quest series, but they only included a minor reference to the character.