Adult Mag appears throughout the Wild Arms series, first time in Wild Arms 2 as an item, and in later installments as an optional boss.
Wild Arms 2
Adult Mag appears as an item that can be found in Slayheim Castle. Inside the library is a blue book on top of one of the shelves. If Brad kicks the shelf, it will fall down. Another bookcase in the room happens to have a missing book, which happens to be the Adult Mag. Upon putting it back into place, the bookshelf will slide over to reveal a hidden room, wherein the Secret Sign item can be found.
Wild Arms 3
Adult Mag can be found inside the Leyline Observatory, at the 3rd floor. In the room at the very back, examining the bookshelf will reveal a book that requires a Duplicator to open. If the group has a Migrant Level of 18+, an optional boss encounter can be triggered, having the group fight against 12x Adult Mag enemies at once.
Wild Arms Alter Code: F
Known as Dirty Mag or Sukebe in this game, this optional boss is possible to fight on the 2nd floor of the Sea of Wind Epitaph. If you examine the bookcase with all males in the front party - Rudy, Jack, and Zed - you will be asked if you wish to read the Dirty Mag. This will trigger the optional boss fight against 12x Sukebe enemies at once.
Wild Arms 4
This does doesn't reward you with any special items upon defeat, although it grant you a sizable amount of both Gella and EXP.
Wild Arms 5
Adult Mag can be found in the remains of Clive Winslett's house, and can be fought if the first three characters in the part are all male - Dean, Greg, and Chuck. This will trigger the optional boss fight, putting you up against 8x Adult Mag enemies.
Upon defeating all of them you're rewarded with the Night Princess badge.