This is a list of theories that have to do with The Legend of Zelda, The Legend of Zelda: Oracle of Ages, and The Legend of Zelda: Oracle of Seasons together.
It is possible that the Japanese for Darknut, タートナック (Tātonakku), was intended to represent the English words "Dark Knight," especially considering the nature of the enemy in question and Japanese conventions for representing English with katakana. The localization team may not have realized what the katakana was attempting to borrow from English, with the series recycling the original mistranslation throughout the games rather than correcting it. However, this raises the question of why ダークナイト (Dākunaito) was not simply used instead, as it more accurately represents "Dark Knight" phonetically.
It should also be noted that Darknut and Iron Knuckle アイアンナック (Aiannakku) share the same katakana ending. This poses further questions as to the translations of both names: Darknut could have been "Dark Knuckle", and Iron Knuckle could have been "Iron Nut" or "Iron Knight".