On a sprite sheet for Mrs. Puff (atlas0) in the Krusty Krab, there is a creepy humanoid character on the right with a hyper-realistic mouth, popping eyes, and ears, whose section in the sprite sheet is named "t_tmpYummer". A separate pose of the character's hands are on the atlas2 sheet.
The original creator of this character, a developer from Seven2, has come forward and confirmed it was made around 10 years ago as a modeling test. They kept the model around and it was an inside joke around the office. They used it as a placeholder so it was very recognizable as needing to be replaced in their work. The devs wanted to highlight as many moments from the show as possible to honor the series, and explored what it would be like in the game if a character would peek around a pole behind Mrs. Puff or behind Mr. Krabs (It was undecided at the time, and the character was also undecided), but decided against it in the end. They forgot to remove him from the atlas and the rest is history. His name is Yum (Yum, not Yummer) because yum is the exact opposite of what you would use to describe him.
In one of the sprite sheets for outside of Sandy's Treedome, there is a sprite for a destroyed version of Sandy's suit. It was to be part of a puzzle where SpongeBob would need to wear one of the water helmets, and it acted as a clue.
Symbol Name
Sprite
t_suit1
t_suit2
By modding the game, it is possible to create the Suit object in-game. It uses just the name "suit" in the code.
All code for it is complete besides creating it in the scene. It uses the otherwise unused SuitDialogue text.
There are sprites of SpongeBob's walking pose that are the full design of him in that pose. In-game, walking SpongeBob is made of multiple separate parts. These were probably just used as references.