The Golden Fleece in the story of Jason and the Argonauts: It's an alternate name for a MacGuffin.
The Finnish national epic, The Kalevala, revolves around various people searching for and fighting over The Sampo, which is eventually lost at sea. The standard Kalevala compilation by Elias Lönnrot describes it as a mill which produces gold, wheat and salt, but he made this up - his original sources never specified what it was and nobody really knows to this day.
Several of the Labours of Hercules boil down to "kill dangerous creature(s)", "capture dangerous creature(s)", or "acquire object(s)".
Killing the Nemean Lion. While the lionskin grants the power of invulnerability, and later is worn by Hercules, its powers aren't relevant to the rest of the story.
Capturing the Ceryneian Hind (a.k.a. the Golden Hind). Any object whose capture / theft would have offended a deity would have served. Turns into a case of No MacGuffin, No Winner.
Capturing the Erymanthean Boar.
Capturing the Mares of Diomedes.
The girdle of Hippolyta. Its magic powers, if any, aren't relevant; neither is its possible status as a symbol of authority, because the attack by the Amazons was instigated by the false belief that Hercules was engaged in a kidnap attempt, not by anything to do with the girdle itself.
The apples of the Hesperides. They just have to be retrieved.
The Golden Apple of Discord. A golden apple that says "For the Fairest", Eris created it so that Hera, Athena and Aphrodite would fight over it. It eventually led to the Trojan War.
The Holy Grail. Much of Arthurian legend concerns different knights' quests for the grail, but once the grail is found, the court of Arthur has nothing better to do and is left to disintegrate. It has in fact first been mentioned in a novel about the Arthurian legend.
The sacred scrolls of the journey to the west are the most literal Mac Guffins, as the scrolls are completely empty and worthless by itself and according to the myth, they only gained value because people believed they are valuable.