Chouchou

Chou-Chou (シュシュ Shushu) is a white dog from Orange Town who guards a pet shop, day and night, unmoving, because the shop belonged to his deceased owner, Hocker.

Buggy arc
When first meeting Luffy, Chou-Chou swallowed the key to the cage Luffy was in at the time to which he was half-strangled by Luffy trying to get the key back. Then Mohji and Richie appeared to search for Luffy, Zoro, and Nami. When Richie spotted the pet shop they headed for it to get a quick snack. Chou-Chou fought against Richie to defend the shop, but got beaten. In the end, he could only watch his master's pet shop burn to the ground. Luffy, out of his cage thanks to Richie, saw how sad the dog was for the loss of his only treasure, and defeated Mohji and Richie as payback for Chou-Chou. After the battle, Luffy presented Chou-Chou with a bag of dog food, the last one from his master's shop. Chou-Chou later helps the trio escape from the angry townspeople after Luffy reveals that he was the one who knocked out the mayor, as well as being a pirate. Chou-Chou is last spotted in Buggy's mini story in volume 5. Chou-Chou is shown biting Mohji on his rear end.

Trivia

 * Chou-Chou's unwavering faithfulness to his master is based on the concept of a faithful dog remaining loyal to their master despite their master's death. This concept is commonly seen in various multimedia and was made famous by the devotion of real life dogs like Hachikō and Greyfriars Bobby.
 * Seen from the front he looks almost exactly like Tintin's dog, Snowy.