This is all the beta elements of New Super Mario Bros: The Underground.
BossesApparently, there was going to be two other Princesses who were also going to be kidnapped by relatives of Bowser. Sakki who would get kidnapped by an unnamed Space Robot Bowser, and Gluebia, who would be kidnapped by a Rubber Band Bowser. There is a model of the Rubber Band Bowser, but it was never used. There were also many, many, beta desgins for Tom Deviling. Some had him as an mutant, and some didn't even have him as human.
Orginally, a Blue Jay suit was going to be in the game, but it was taken out for unknown reasons. As a joke, one of the modelers made an Superman suit, but it was never used. There also was a Goo Suit, which was actaully was in the game in the beta, but tooken out for various reasons.
There seems to be only two missing enemies, Old Bro and Ojay. Old Bro was never used for unknown reasons, while Ojay was taken out for being "too distrubing". Ojay was an elphant made out of various parts of the level, and would often chase Mario.
Two levels were taken out. One was a test level, and featured a Blue Hill background, unlike seen in the game. The other was a Space Stage, peferably used when Sakki was still in the game.
There was supposed to be a Robot World, as well as an another Underground one.
There was an extended theme for Asokos that was never used, as well as an Ice Song made out of ice effects.