Deep within Super Mario Bros. 3 lie hidden, unused levels. Some of them seem polished, with working goals and plenty of enemies, while others are devoid of enemies, lacking exits, or too short to be full levels. Whether they evolved into other levels, or were merely used for testing, the "lost levels" provide an interesting glimpse into the development of Mario 3.

Lost enemies
Gold Cheep Cheep animation Green Parabeetle animation
The lost levels contain two enemies not present in the regular levels: Gold Cheep Cheeps, and Green Parabeetles. The Gold Cheep Cheeps come in groups and swim faster than regular Cheep Cheeps. The Green Parabeetle is basically a red Parabeetle that flies faster.

Our very own MEGAߥTE made a ROM patch that allows easy access to the lost levels -- if you have the SMB3 ROM. World 1's map screen is replaced by a custom map, with pipes leading to the levels so that you can re-enter them as many times as you wish.

Source: Classic Gaming

You really should check that out if you want to take a trip down memory lane... and it's strange they didn't include the levels because some of the look awesome.