big dummy

The only place official artwork of a Block Boo in Yoshi's Story appears is in the 1998 German official guide for the game, being absent from all other guides. The artwork is notable for seemingly depicting the Block Boo mid-transformation, being neither fully round nor an actual block at that point.

Source: SCANdalDE

In Paper Mario: The Origami King, a hallway in Bowser's Castle contains several atmospheric paintings of Bowser, as well as one of King Olly. Here are the textures for the paintings, extracted from the game's files.

Source: noclip.website

A small reference that is easy to overlook when playing the game over a longer period of time instead of in one sitting occurs in Super Mario 3D World.

The first Mystery House level in World 2 is called "Mystery House Melee", while a similar level encountered much later in the game in World Mushroom is called "Mystery House Brawl".

These are obviously a reference to Super Smash Bros. Melee and Super Smash Bros. Brawl, but this is only clear when putting the names next to each other, and would be easy to miss if the player no longer remembers the first level when coming across the second.

Reader-submitted drawing of Mario and Peach featured in a 2003 issue of the Japanese Nintendo Dream magazine.

Officially licensed 2018 Goomba hand puppet.

In Super Mario Galaxy 2, the large planetoid in Rolling Masterpiece Galaxy is using a very blurry image of Earth as its reflection map. This is actually "The Blue Marble", an extremely famous photo of the Earth from space taken on the Apollo 17 flight to the Moon in 1972.

Unused rectangual cuboid-shaped tree found in the files of Super Mario Galaxy.

Luigi and Daisy illustration featured in a 2004 issue of the Japanese Nintendo Dream magazine.

Mario Artist: Polygon Studio was an entry in the Mario Artist series for the Japan-only 64DD add-on for the Nintendo 64 that allowed users to create their own 3D models and insert them into scenes.

In a 2003 issue of the Japanese Nintendo Dream magazine, a tutorial was published on how to create a model of a specific Shy Guy, who has been a popular recurring character in the magazine's long-running "Dream Drunker" comic (famous for his gimmick of never saying anything except variations of "HO~").

A reference image of the Shy Guy riding a minecart and the finished model are provided. Note that the model also includes one of the exercising men from the opening of Mario Paint, who also appear in the Mario Artist games.

Splash page illustration from the 2013 Nintendo Japan company booklet, featuring Flying Squirrel Mario.

Unused sprites of Mario (near and far planes) being squished, found in the data of Mario Clash for the Virtual Boy. Interestingly, this includes a gentle unsquishing, which suggests that this was not meant to be something that would hurt Mario.

Hungarian print ad for various sizes of Mario and Yoshi plushes.

Photo from a promotional 1995 Rare leaflet showing an artist at work creating a Killer Instinct/Donkey Kong Country 2 render at a Silicon Graphics workstation.

Plushes advertised in a 1999 Nintendo Power catalog.

Board game pieces for cutting out, featuring Mario, Luigi, Peach, Bowser and the Shogaku Ichinensei magazine's bowtie-wearing Yoshi design, from a 1991 issue of the magazine.