Guardians of Terra

Chaos Guardians in Final Fantasy IX are four powerful monsters each representing an element. They are based on and directly named after the original Four Fiends of Final Fantasy, and are called Maliris, Tiamat, Kraken, and Lich. The Chaos Guardians are based on the the four guardians of the elemental shrines on Gaia that seal the entrance to Terra. The four appear very similar to the Chaos Guardians and bear the same elemental affinities.

Story
Zidane's party learns from Hildagarde Fabool that the man they pursue, Kuja, hails from another world: a world called Terra. To open the portal there, the party must place an elemental mirror in each elemental shrine. The party splits into four teams of two to tackle all four shrines simultaneously.

The Earth Shrine is a shrine built to house the Earth Mirror, located on the Outer Continent. Zidane and Quina meet the Earth Guardian here, whom Kuja has warned of their arrival. After they defeat the guardian, Zidane and Quina release the Earth Seal.

The Fire Shrine is built to house the Fire Mirror, located on the Lost Continent in an active volcano. Freya and Amarant are sent there and encounter the Fire Guardian whom they defeat. The Water Shrine is built to house the Water Mirror. Garnet and Eiko are sent there and encounter the Water Guardian whom they defeat and release the Water Seal. The Wind Shrine is built to house the Wind Mirror, located on the Forgotten Continent in a valley filled with high winds. Steiner and Vivi are chosen to tackle it, and they meet the Guardian of Wind. After they defeat the guardian, Steiner and Vivi release the Wind Seal, thus allowing access to the world of Terra via Shimmering Island.

After Kuja has decided to destroy the world, he recreates the four guardians from the memories of the Crystal in Memoria to try and kill Zidane and his friends as they track him down. Crystalline versions of the Chaos Guardians appear in the Crystal World.

Gameplay
The Earth Guardian is the only one of the four shrine guardians to be fought as a boss, while the other three are killed off-screen. Only Kraken goes unseen prior to entering Memoria.

The Chaos Guardians—Maliris, Tiamat, Kraken, and Lich—are fought in Memoria, and much like in the original Final Fantasy, their fights are triggered by passing otherwise unmarked areas in the dungeon. Crystal palette swaps of the four are fought in the Crystal World, the game's final area.

Data hacking shows that there are many gaps in the numerical sequence of battle formations and monster IDs, showing that many battle formations and monsters were deleted prior to the final version. In addition to many more, three missing monster IDs appear around the "Earth Guardian", making one suspect that in the original version the player was to fight all four of the guardians instead of only one (in the final game, the other three are defeated via cut-scenes). Design artwork for these other three guardians is known, as well as deleted locations, which would probably have been where they were fought.

Trivia

 * The Four Fiends is a recurring concept in the Final Fantasy series as four demonic enemies that serve the game's main antagonist and are fought as bosses. They are often associated with the four elemental forces—Earth, Fire, Water, and Wind, and can be seen as counterbalances to the four elemental Crystals, as the Four Fiends often seek to subvert the influence the Crystals have on the world.