The Legend of Zelda
February 21, 1986
Digdogger, the boss of Level 5 (Level 2 in Second Quest), is stated in the Japanese game's manual to be a giant Unira, the urchin monster from Clu Clu Land (much like Manhandla is based on a Mario enemy). Having only one large eye, it very much resembles the Boss Unira from Vs. Clu Clu Land. Additionally, to defeat it, Link must play the Recorder first to shrink it, which may reference how in Clu Clu Land the game's protagonist Bubbles could stun the Uniras by using a Sound Wave.
This reference was removed in the international releases of the game, as well as Japanese re-releases and remakes, where it was simply called "giant sea urchin".
Along with all the bosses from the original Zelda, Digdogger also returns in The Legend of Zelda: Oracle of Seasons, but it's defeated in a different way.
Additionally, the Rupee in The Legend of Zelda is created using the same sprite as the gold bars from Clu Clu Land. While all Gold Bars are orange in the original game, Rupees may be either orange (worth 1 Rupee) or blue (worth 5 Rupees).
Despite looking identical, they're not considered the same object, but rather different objects designed in the same way for technical reasons.
Copyrights for both series are held by Nintendo.