A little digging into the SMW2 ROM uncovers some interesting sprites that did not appear in the final game.
On the top-right are Shy Guys and a Tweeter from Super Mario All-Stars, and some odd-looking Bob-ombs above the green X's. On the left side we can make out three more possible Yoshi transformations: Plane, Tree, and Mushroom. It's possible these were planned but scrapped, or used for testing purposes. If they actually made it into the game, here's our best guess at how they would be animated:
Yoshi's Island had its fair share of monkeys, but at least one was left out. RaccoonSam sent us these unused sprites that, when assembled, form Donkey Kong in the style of Yoshi's Island's monkeys.
RaccoonSam also found another unused mushroom and a sad-faced rock.