The Zou Zou no Mi, Model: Mammoth is an Ancient Zoan-type Devil Fruit, which allows the user to transform into a mammoth hybrid and full mammoth at will. It was eaten by Jack. According to Nekomamushi, it is the ancient form of the Zou Zou no Mi.
The full mammoth form granted by this fruit gives the user a massive frame, significantly increasing Jack's already abnormal size. Jack can level buildings with a single swing of his trunk and leave imprints of his feet on the ground just by stepping on it.
However, the user is extremely heavy and slow in this form, leaving them unable to dodge high-speed attacks. Otherwise, the user is affected by the standard Devil Fruit weaknesses.
Jack has used this fruit to improve his combat capabilities; he can cause massive destruction with a single attack and bulldoze nearly anything that gets in his way by charging forward. So far, his only displayed attack has been swinging his trunk to damage his opponents and surroundings.