In the recent episode of One Piece, the character that has been a blond haired Mammoth from the start of the arc transformed and we finally got the more info about him.
After he took a blow from Cat Viper, he came into rage and transformed into a gigantic human and we found out some new stuff about him.
- Jack is the right hand man of Kaido who is one of the yonko. This explained Kaido walking behind a mammoth in the new opening of the anime.
- He supposedly consumed a Mythical Zoan devil fruit that turns him into a huge Mammoth with great strength and maximum endurance. If his size is compared to Duke Dogstrom, he is humongous as the Duke is himself huge as compared to humans.