Tony Tony Chopper is the doctor of the Straw Hat Pirates. He is a reindeer that ate the Hito Hito no Mi and comes from Drum Island. He is the sixth member of the crew and the fifth to join.
Most of the time, Chopper is a toddler-sized human/reindeer hybrid, his devil fruit abilities allow him to change his appearance depending on the situation. Chopper's left antler has a metal plate wrapped around it at all times, from being broken in a fight when he was younger. The usual clothing he wears include a pink, fuzzy top hat with a sideways medical cross (given to him by Hiriluk) and a purple pair of shorts. Chopper also has a remarkable feature that no other reindeer have, and that is a blue nose, while normal reindeers have black ones. As Oda's style evolved, Chopper has been given a more "chibi" appearance. This means that when in his Brain Point, Chopper has a larger head and eyes.
A running gag is that various characters in One Piece think Chopper is a tanuki (raccoon-dog, often simply translated as "raccoon") whilst in his Brain Point form. This is a play on the similar word "tonakai," the Japanese word for reindeer, from which the "Tony" in Chopper's name is derived. His heavy point form likewise is mistaken for a gorilla or an abominable snowman.
Chopper is the inventor of a pill called the rumble ball which gives Zoan devils fruit users the ability to change into more forms. The form he maitains most of the time is his Brain Point where he takes the form of a toddler sized human/ reindeer hybrid. Brain Point allows him to become smarter and by "scanning" with his hooves he is able to find an enemy's weak spot. His next most common form is his Heavy Point where he transforms in to an almost gorrila looking human/ reindeer hybrid. In Heavy Point he becomes heavier and gains strength all throughout his body. His third most common form is his Walk Point in which he looks like a normal reindeer. In this form he is faster and more agile. When chopper consumes a rumble ball he can change into 4 more forms. These forms include Horn Point, Jumping Point, Arm Point and Guard Point. In Horn Point form his horns become massive and his legs become more powerfull so he can charge at enemies and do serious damage. In Jumping Point Chopper gets taller and gains strength in his legs so he can jump extrordinarily high. In Arm Point his biceps and triceps become massive and extremely strong. Also instead of hands on the end of his arms he has hooves so his attacks have a greater impact. Last is his Guard Point in whice he transforms into a giant ball of fur that protects hi body from any attacks, except for his face which is completely exposed. Unfortunatley the rumble ball does have its faults. the rumble ball once consumed gives the user the ability to change into more forms but is only lasts for three minutes and no more can be consumed(safley) for 6 hours. When Chopper fights Kumadori in the Water 7 arc he is forced to consume more than one. After he ate a second rumble ball he still had acess to his other forms but he could not control the and it made it difficult to fight which resulted in his sustaining many injuries. After the second was enough Chopper was forced to consume a third. After the third rumble ball is consumed Chopper completly loses control of his body and mind. He becomes a giant, long haired, sharp clawed, roaring monster that can't distinguish the difference between his friends and his enemies. This form lasts for several hours and creates too much distruction for him to use unless it's a dire emergency( such as the Straw Hats fight with the real Kuma on Shabondy Archipelago where his only desire was to save his nakama).