Board Thread:Questions and Answers/@comment-30372146-20161031002734/@comment-26456251-20161113234407

It is confirmed and canon it's a cape; the sprite proves it's a cape, besides, a scarf has an end, a cape doesn't, and it has no visible end. It's short, but that's probably because he used it as a full sized cape when he was smaller (a child) and wears it still because he likes it.