The enemy "Assault Rider" was meant to appear in the game but its style didn't fit any of the worlds so it was removed and used in Kingdom Hearts 2 instead.
Completing the world "Deep Jungle" before Wonderland will cause Snow White to be kidnapped instead of Alice in the cut scene which is shown after completing Deep Jungle.
In Deep Jungle, outside of the tents on a table is a pot and cup which bare a resemblance to Ms. Potts and Chip from Beauty and the Beast. These characters also appeared in the Tarzan film.
There was a planned boss battle with Riku who would be possessed by Ansem in an area titled Crumbling Island at the End of The World which was based off of Destiny Island, but devoured by the darkness.
Contributed by ST3
If you enter the Hercules Cup time trial, get to Hercules, pause the game and choose to restart the time trial, it will take you back to the start of the Hercules fight and the timer will be frozen at 00'06"23.
In the Japanese version of Kingdom Hearts, the Chernabog fight does not have the "Night on Bald Mountain" music. Instead the regular boss music will play. The "Night on Bald Mountain" music was added to the international release to make the scene more true to the scene from Fantasia. The music was added into the Final Mix version of the game.
Rikku was going to have a major role in the first game. She was going to be a part of Leon's group in Traverse Town and help Sora throughout the game. She was removed because of the similarly-named character Riku, and was replaced with Yuffie. She later appeared with the rest of the Gullwings in Kingdom Hearts II.
The Pride Lands were originally meant to appear in the game, but were taken out due to the game engine not being considered for Sora's lion form, with four-legged movements, and his fighting style being too complicated. Simba was turned into a summon for the first game and the Pride Lands were put into Kingdom Hearts 2.
Disney Castle was suppose to be playable. It was shown in a beta trailer but the level was cut from the final version. However, through cheat codes, it can be accessed.
The secret ending, "Another side, Another story," was not meant to be a real game. It was only there to show off ideas that Tetsuya Nomura had for the sequels. It was not until Kingdom Hearts 358/2 Days that the fight shown was playable.
Bahamut, a recurring summoned creature from the Final Fantasy series, was planned to be a summon for Sora. But it was removed from the final version. Searching through the game data, a Bahamut summon option can be chosen through the use of cheat codes. However, since there is nothing to execute when it is chosen, the game will crash.
Contributed by zealk
According to some concept art released by Square Enix, Sora was originally not going to wield a Keyblade at all, but a chainsaw sword instead. He was also going to have a design with lions fur, claws, a tail and ears.
Sora was not originally the main protagonist of the series. In the beginning, Disney had desired Donald Duck to be the protagonist, while Squaresoft wanted Mickey Mouse. Series creator Tetsuya Nomura wanted neither and developed an original character based off of Disney's style. In the end, Sora was created. His yellow shoes, red shorts and white gloves are based on Mickey Mouse.
Contributed by bksonic
In the Secret Place, on Destiny Islands, the walls are covered in paintings which seem to foreshadow some of the earlier events in the game. For instance, there is a painting of Sora winning at the coliseum; the tree house from Deep Jungle; and Pluto waking Sora up in Traverse Town.
Contributed by RayMonkay
Before the game was released in the U.S., Square held a "Name-in-Game" contest. A person named Kurt Zisa won the contest, and his name was used for a hidden boss in the game.