The Minion who carries the zodiac stone in Minions: The Rise of Gru is Otto.
After Gru steals the stone from the Vicious 6, he gives it to Otto for safekeeping while he returns to his basement lair. Otto is supposed to protect the valuable artifact, but he trades it to a biker in exchange for a pet rock. When Otto realizes that the rock was actually the zodiac stone, he sets off alone to recover it.
That mistake becomes one of the film’s main comic plotlines. Otto follows the biker across the desert and eventually befriends him, while Gru and the other Minions search for the missing artifact in San Francisco. The stone is important because it can transform people into powerful animal forms associated with the Chinese zodiac.
Bob, Stuart and Kevin also become deeply involved with the stone’s adventure, which is why viewers sometimes remember one of them as its main carrier. However, Otto is the Minion entrusted with it and the one who pursues it after accidentally trading it away.