Pre-release and unused content

Before a game gets released you might see enemies and places that aren't in the released game, because they are taken out for unknown reason. The elements that were once in the game but wer're taken out are called Beta Elements.

Beta Elements
Here is a list of Betas Elements appearing in Mario games:

Super Mario Bros. 3
There were going to be two new enemies called Gold Cheep Cheep (a golden version of a Cheep-Cheep, and Green Parabeetles (Green colored Parabeetles). The Gold Cheep Cheeps would come in groups and swim faster than regular Cheep-Cheeps and the Green Parabeetle is a green parabeetle that flies faster than normal ones.

Paper Mario
In one screenshot Poochy was going to appear. It is rumored he was going to be a partner for Mario.

Mario & Luigi: Superstar Saga
Wario, Fox McCloud, Captain Olimar, Samus, an Excitebike Racer and Link we're going to be in the Starbeans Cafe after making cofee, instead of E. Gadd.

Paper Mario: The Thousand Year Door
There were many unused badges. See Badges for information on them. The partners from Paper Mario were going to appear but we're cut-off as well. Bow and Parakarry were the only ones left.

Mario & Luigi: Partners in Time
In this game, there were going to be some Blooper enemies appearing in the Vim Factory wearing red shoes. While the screenshot only showed Luigi attacking one, it isn't known what their attacks were.