A huge tan walking thing
Well, according to Nintendo (and infrences) that thing actually spews Widgets, as seen in prototype screenshots. From this, we can infer that:
1. Killing the thing would give you a shine sprite.
2. Shadow Mario was implented in the game, but sometime after the "alpha".
3. The thing would spew goop in the "alpha". There is proof behind this claim.
4. This is not a bump. If it was May 13, it would be.