There it is, as predicted: Yoshi’s butt is in Scribblenauts Unlimited — along with the rest of the dinosaur, the Mushroom Kingdom troupe, and and the Zelda gang. The game will have “dozens of characters and items” (e.g. mushrooms, karts) from Nintendo’s two big franchises.
The text says this is Wii U-exclusive, which is odd. I didn’t expect it on the PC version, but what about 3DS?