From the Super Mario Wiki
The Mad Adder is a giant mechanical snake that lives in Smithy's Factory during the events of Super Mario RPG: Legend of the Seven Stars. The snake is also the pet of Domino, a magic-wielding jester and member of the Smithy Gang. The Mad Adder's main attack is poisoning Mario and his allies.
During the battle with Domino and his partner Cloaker, Mario and company can choose which one of the villains they want to defeat first. If the sword-wielding Cloaker is defeated first, Domino will retreat to his pet, the Mad Adder, and electronically fuse with the giant snake. However, although the Mad Adder and Domino combined their power, they are still no match for Mario's party and are quickly defeated.
The Mad Adder is a palette swap of Earth Link, Cloaker's snake.
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