Bagan

The phrase "Better luck next time, buddy" best describes Bagan, a creature intended to be used in a handful of Toho daikaiju movies, none of which ever came to be: the original draft of The Return of Godzilla, Mothra vs. Bagan and Godzilla vs. Bagan, before finally appearing in the video game Super Godzilla. The Bagan in The Return of Godzilla is completely different from the later version of the beast, which would stay more or less the same in each of its subsequent "appearances."

Version #1 (The Return of Godzilla)
The first version of Bagan was a shape-shifter. It started out as an ape-like creature but after being wounded it became a dragon-like creature. Each time it changed it healed itself. It dives into the sea and becomes fish-like. Bagan runs into Godzilla, but finding that it can't defeat him in any single form, it makes a towering, combined form. But after this, it can no longer change shape, meaning it can no longer heal itself, allowing Godzilla to defeat it.

Version #2 (Mothra vs. Bagan, etc.)
In Mothra vs. Bagan, Bagan is an ancient creature that protects the land of China. He was frozen in the Himalayas long ago but now, due to global warming, the beast breaks free. He sees the damage humans have done to the land and begins to attack them, but Mothra intervenes. As the fight between the two monsters takes them to Singapore, Mothra's egg hatches and the larva arrives to help its parent. Despite their combined attempts to stop the creature, Bagan kills the adult Mothra and moves on. The larva cocoons itself and becomes an adult Mothra, attacking and defeating Bagan in Hong Kong. Unfortunately, after Godzilla vs. Biollante did not perform as well at the box office as Toho had hoped, they decided not to risk making another movie with another new monster, and so Mothra vs. Bagan was canceled.

In Godzilla vs. Bagan, Bagan was intended to fight the King of the Monsters himself, Godzilla. Not much else is known, other than the Gotengo was also supposed to fit into the story somehow. Unfortunately, Godzilla vs. Bagan never got past the development stage.

Finally, Bagan received a starring role as the final boss in the video game Super Godzilla. Supercharged by cells from Godzilla and King Ghidorah that his alien commanders had spliced into him, Bagan is easily the most powerful foe in the game, generally seen as impossible to defeat as normal Godzilla, and still seen as a difficult fight as Super Godzilla.

Powers
The only known powers for the first, Return of Godzilla version of Bagan are shape-shifting and ability to heal himself.

In the storyboards for Mothra vs. Bagan, Bagan is shown to be blowing a hole in one of Mothra's wings with an energy beam from his mouth. He is also shown to be firing some sort of lightning bolt from his main, nasal horn, and growing what seems to be some type of energy-wings out of his back.

The version of Bagan that appeared in Super Godzilla possessed insanely high physical strength and durability. He could slash with his powerful claws ("Slasher Claw"), perform a Diamond Storm (in which he would shoot white, star-like objects from his horns), fire a white Plasma Beam from his mouth and generate a Force Field to block incoming attacks. Bagan's attacks were, bar none, the strongest used by any enemy in the game. In addition, he had the most health out of any enemy in the game and Godzilla's attacks would inflict barely any damage, making it necessary to transform into Super Godzilla to defeat Bagan and complete the game.

Trivia

 * Bagan01.jpg is the only known monsters to be more physically massive and greater in weight than the final form of Biollante, who weighs in at a "mere" 220,000 tons.
 * If including any Godzilla media, Bagan is the tallest monster (150 meters), second place going to various Ghidorah incarnations, including Keizer Ghidorah.
 * Bagan was mentioned in an interview for Godzilla: Unleashed. He was one of several monsters considered for the game, but was dropped in favor of more obscure monsters (Varan being explicitly stated).
 * In M.U.G.E.N, you can play as him. He is one of the customizable characters in the game. There are two versions.
 * Vlcsnap-900495.png video, supposedly consisting of cut footage of Bagan from Godzilla: Final Wars, was uploaded to YouTube on June 19th, 2012. However, the footage turned out to be a hoax, showing off the monster called Reizaus from the Japanese show Super Fleet Sazer-X.