Shiro

is a dog from Jiraiya's novel, who appears within Tsunade's dream world as Sasuke and Itachi Uchiha's pet.

Background
Shiro was originally owned by a young boy and his father in Konohagakure. As a pup, he was seen about to drown in a river until Naruto Uzumaki came to rescue him, with the assistance of his father Minato. Sometime after the incident, he became Sasuke and Itachi Uchiha's pet.

Personality
From what has been seen, Shiro appears to be a very happy and jumpy dog.

Appearance
Shiro is a small dog with small black eyes and a small black nose. He has a light brown upper body and tail while his under-side is white. He has a small, circular white spot above each eye and his eyes are outlined with a thin white colour.

Jiraiya Shinobi Handbook: The Tale of Naruto the Hero
Shiro is seen with Itachi and Sasuke, when Naruto arrived at their house. Later, Shiro is seen with Sasuke, as he recalls a memory where Naruto saved Shiro before. Two years later, before Sasuke leaves Konoha, he gives Shiro a good-bye.

Trivia

 * When written as "白", "Shiro" means "white", while "城" means "castle" and "代" means "substitution".