The Mahogany Brain is an item referenced by Aramis. It is an actual item within the game. It is found equipped on Aramis' special zombie and can be stolen along with the rest of its unique items. In Etna Mode, the Mahogany Brain (and its associated items) can be found equipped on a Zombie on replays of Stage 3-3 (Etna Mode).
The Mahogany Brain increases a character's INT , making it a decent accessory for early Mage-types, only to be overshadowed by late-game gear.