Forgotten Characters
This character's only mainline playable Mario Kart game was Super Mario Kart
DK Jr.
This game's commercial failure is commonly blamed for shutting down fan favorite studio AlphaDream
Mario & Luigi: Bowser's Inside Story + Bowser Jr's Journey / Mario & Luigi: Bowser's Inside Story 3DS remake
Mega Mushroom - Mario Kart Wii
This seldom seen enemy features as a stage hazard on Peach Beach
Cataquack
This 3DS sports title flopped hard. Unfortunately, it is still the last appearance of "Mario Baseball" to date.
Mario Sports Superstars
Luigi's Mansion 3 - E Gadd's Laboratory / E Gadd's Theme
This goomba variant appeared in the Super Mario Land series
Goombo
This game is the seldom discussed sequel to a popular Mario puzzle game. It added a 4 player VS mode where you could play either 2v2 or FFA.
Dr. Mario 64
Mario Superstar Baseball - Menu / Character Select
These 2 characters were both prominently featured in the Super Mario Galaxy movie. They also have both only been playable in a single mainline Mario Kart title.
R.O.B. and Honey Queen
This game released as a launch title for the Virtual Boy
Mario's Tennis (I will not be accepting Mario Tennis, that is the N64 one. I apologize in advance)
Wario Land Shake It - Stone Carving City / 1-1
This was the final Game & Watch system ever produced. It had identical gameplay to "Ball" but featured a Mario themed reskin.
Mario the Juggler
Mario Party 9 - Bowser Jr's Mad