Rumble Ball

The Rumble Ball is something Tony Tony Chopper made through research (and a bit of an accident, according to the Anime). The "Rumble Ball", when eaten, gives Zoan-type Devil Fruit users several more transformations for a three minutes by 'distorting the wavelengths of transformation'. For Chopper, if comes with 4 more transformations:

First Appearance: Chapter 148; Episode 88

It's likely that Chopper will discover other forms that he can change into. It has recently been revealed Chopper can't eat multiple Rumble Balls back to back within 6 hours or otherwise he'll lose control of his transformations.
 * Jumping Point (飛力強化 (ジャンピングポイント)): Chopper keeps his reindeer legs in a human-like form, which allows him to jump long distances. "Jumping Boost" in the English.
 * Arm Point (腕力強化 (アームポイント)): Chopper's second most powerful form, his fore-hooves become extremely muscular in his humanesque form, powerful enough to inflict a great amount of damage. In the English version, this is called his "Arm Boost".
 * Horn Point (角強化 (ホーンポイント)): Similar to Walk Point, but Chopper's forelegs get bulkier and his horns get larger and sharp. In the English version it is called "Horn Boost". Chopper said that it was his strongest point (besides Monster Point).
 * Guard Point (毛皮強化 (ガードポイント)): Chopper's fur grows out and becomes an impenetrable shield against outside attacks. He looks like an overgrown Chia Pet in this form. In the English version it is named "Guard Boost".

In addition, if he were to eat 3 Rumble Balls in a row, he will turn into Monster Chopper, clearly his most powerful form. See the main article for more info on Monster Chopper.

It is not yet known exactly what is in a Rumble Ball that makes it so unique.

It's unknown if other Zoan users can also be affected by the Rumble Ball since so far only Chopper ate the Rumble Ball.