Lord Jabu-Jabu

Lord Jabu-Jabu is a recurring character in the Legend of Zelda series. He has the appearance of a giant fish, and is worshipped by the Zora race as their patron Deity. In both of Lord Jabu-Jabu's appearances, he not only acts as a character, but his belly acts a one of the game's dungeons.

The Legend of Zelda: Ocarina of Time
Lord Jabu-Jabu resides in Zora's Fountain, where he is cared for by the Zora. After King Zora XVI refuses to give Ganondorf the Zora's Sapphire, he places a curse on Lord Jabu-Jabu, causing him to act strangely, and swallow his caretaker, Princess Ruto, while she was bringing him food, along with the Zora's Sapphire. After recovering a note from Ruto to her father, Link is granted access to Zora's Fountain. On the altar of Jabu-Jabu, Link presents an offering of fish to the deity. Opening his mouth to devour the fish, Jabu-Jabu sucks Link into his belly, inside of which Link encounters Princess Ruto. Ruto reluctantly allows Link to aid her and they eventually defeat the parasitic Barinade and recover the Zora's Sapphire. Afterwards, Jabu-Jabu will open his mouth whenever Link approaches, allowing him to enter the dungeon at any time when he is a child.

Sometime during the seven years Link is trapped in the Sacred Realm, Lord Jabu-Jabu disappears from Zora's Fountain, and is never seen nor mentioned in the game again. It is widely believed that he either died or migrated when Zora's Domain froze over. It is also believed that Jabu-Jabu may also have been encased in frozen ice, due to the fact that a huge iceberg roughly his size can be seen were he used to be.

The Legend of Zelda: Oracle of Ages
Lord Jabu-Jabu lives in Zora Village. In the Past, Lord Jabu-Jabu has not yet become a fully-grown fish. In the Present age, after Link clears the poison in the waters, Lord Jabu-Jabu has grown. Link must enter Jabu-Jabu's Belly, which serves as the game's seventh dungeon.

Theories
There has been speculation that The Legend of Zelda: The Wind Waker's Jabun may be Lord Jabu-Jabu or a relative, due to his similar name, appearance, and music that plays while around both Jabun and Lord Jabu-Jabu. Another similarity is that Jabun knows the ancient Hylian Language which was used during the events of Ocarina of Time. Though not affiliated with Nintendo, the Prima's guide for The Wind Waker, states that Jabun was once referred to as Jabu-Jabu, indicating that he survived the future events of Ocarina of Time and returned to Hyrule before taking on the name Jabun following the Great Flood.