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Chopper has eaten a Zoan-class Devil Fruit known as the Hito Hito no Mi, which allows him the ability to transform into a human and human-reindeer hybrid at will.
In addition to his three forms granted by his Devil Fruit, Chopper has also invented a yellow jaw-breaker like drug called the “Rumble Ball”, through some research.
This drug has the power to distort the wavelengths of the Devil Fruit’s transformation, allowing him access to additional transformations for three minutes. However, due to the risk of this alteration, the Rumble Ball’s consumption must be closely monitored, and for this reason Chopper cannot consume more than one every six hours. If he takes a second one, his transformations would become out of control. And if a third one is taken, Chopper transforms into a massive, berserk monstrosity (as witnessed at Enies Lobby and Sabaody Archipelago) with equally monstrous strength that requires lethal amounts of energy to keep active, making Chopper a danger to both himself as well as others.
According to Chopper, during the two years of his absence, he has completely improved all his transformations. Furthermore, he is able to utilize those forms without consuming a Rumble Ball; instead, eating a Rumble Ball now allows him to enter his most powerful form, Monster Point, and fight in it for three minutes. How he managed this is currently unknown.
However, the fruit has granted Chopper more than just the ability to transform; it has also given him human intelligence, which allows him to both speak and think like an actual human. This is what allowed Chopper to become a doctor and learn about medicine in the first place, which is impossible for a normal reindeer.