Nearly two months after Disney launched Urupocha-chan—a tiny plushy that’s modeled after characters like Minnie Mouse and Stitch—in Japan and China, they’re now available to buy in the U.S.

In the now-viral clip, Rocco can be seen trying relentlessly to play with Big Bird—who, while elegant and stoic, doesn't seem overly impressed with his new sibling's energy. The contrast between Rocco ...