As with almost every game in recent memory, somebody is digging through Super Mario Maker to see what lies beneath the game itself. In this case, data miners have discovered a unannounced costume, and you can already test it out providing Nintendo haven't removed the course yet.
This new costume is Mario himself piloting the spaceship from the 1989 Game Boy game 'Super Mario Land', it even includes its own sound effects. The costume hasn't been officially revealed but it looks like its in working condition, so we don't expect it will take Nintendo too long to get around to it.
Nevertheless, if you'd like to give the costume a whirl, you can visit this course [ID 4078-0000-0121-4C73] and have a go.