All of the enemies in Crash Bandicoot 99x were inspired by Looney Tunes side characters. Mr. Crumb was inspired by Count Bloodcount from Translyvania 6-5000, the mummy was inspired by Gossamer, the skeleton was inspired by Satan from Devil's Feud Cake, and while the developer George F. (last name unknown) did not recall the inspiration for the tree, he believed it may have been inspired by Tweety Bird's monster form from Hyde and Go Tweet.

The choice for a horror setting in the game was also inspired by Looney Tunes, particularly the compilation film Daffy Duck's Quackbusters.

According to developer George Fergan, there was a debate within the team on who should be the villain of the LCD handheld games: some wanted to use Mr. Crumb, while others wanted to use Mr. Crool. In the end, they decided to use both for different versions, with Mr. Crumb being used in the more powerful Tiger 99x version and Mr. Crool being used in the cheap LCD version.