In the release build of Super Mario Maker, there were text strings relating to 13 unused Mystery Mushroom costumes:
• BabyMario
• BalloonFight
• EGadd
• GoldenRetri (presumably short for "golden retriever")
• MarioUs (presumably Mario's appearance from what is known in Japan as Super Mario Bros. USA)
• Mashiko (Japanese name of Mary O.)
• MrSaturn (from EarthBound)
• Muncher
• Nabbit
• Popo
• Tetris
• WindowsLogo

Baby Mario, Balloon Fighter, E. Gadd, Mary O., Mr. Saturn, and Nabbit would eventually be added in updates, as would an Ice Climbers costume featuring Popo alongside his friend Nana, while a power-up that turns Mario into his Super Mario Bros. 2 self would be introduced in an update to Super Mario Maker 2.

The most notable names are "Tetris" and "WindowsLogo", two non-Nintendo franchises that otherwise go unrepresented in Super Mario Maker. Tetris was formerly published on handheld and home consoles by Nintendo between the 1980s-1990s, and has been featured through music and Spirits in the Super Smash Bros. series, which is used as the basis for Super Mario Maker's base-game third party costume selection. WindowsLogo seems to be some kind of remnant of a Microsoft Windows operating environment being used to develop the game, as Nintendo has never been involved with the Windows line of software.