Yokozuna

Yokozuna is a giant sumo-wrestling frog, who was a friend (as well as pet) to the legendary shipwright Tom until Tom's execution.

Appearance
Yokozuna is a giant frog with yellow-brown skin and black hair shaped in a samurai-style. He has numerous scars on his belly (from testing his strength against the Sea Train, a task inspired by his comrade Franky's death-defying act). He also has fanged teeth and is seemingly a facultative biped, used mostly for his sumo stance.

Personality
He cares very much for his friends and loved ones. He is exceptionally loyal to Franky who taught him the crawl-stroke when they were younger.

Powers and Abilities
He is a powerful swimmer and he is strong enough to knock out a Sea Train, although it did leave him scars on his face, arms, and belly from challenging it for years. He was also able to block an attack of one of the Just Eleven Jurymen which was able to stop a king bull.

Past
He would always swim through the waters around Water 7, getting in the way of the Sea Train to try and stop it. This habit is said to be caused by Yokozuna wanting to become stronger so he won't lose any more of his loved ones.

Water 7 Arc
In fact, when he was first introduced he was performing this very routine when Zoro spotted him from the Going Merry; after informing the rest of his crew about the giant frog swimming in a freestyle manner not unlike a human, Luffy immediately decided to slightly change course (they were headed to the next island, which happened to be Water 7) to capture the giant frog so his crew could have a nice barbecue meal. This habit also caused Yokozuna to try and stop the Rocket Man while the Straw Hat Pirates, Galley-La foremen, and the Franky Family were traveling towards Enies Lobby, but because Old Lady Kokoro, an old friend of his, was present to tell him of Franky's plight, he ends up helping them in their attempt to get Franky and Nico Robin back.

Enies Lobby Arc
While on Enies Lobby, his defensive style forged through years of facing down Puffing Tom put him in charge of preventing any government agents from getting into the courthouse once the Straw Hats and Co. had breached it, and does this very well, even handling multiple attacks from the powerful Jurors when they arrive. After he becomes exhausted and can't stand up to the them anymore, he is saved by the giants Oimo and Kashi who decide to help him guard the courthouse and aknowledge his prowess. Yokozuna later boarded the Sea Train with the Franky Family, the Galley-La foremem, and the giants and has since returned to Water 7.

Where They Are Now
He was shown later in a mini arc chapter cover entitled "Where They Are Now", no longer challenging the Sea Train.

Trivia

 * Yokozuna means "horizontal rope" and is the highest rank in professional sumo.