It's interesting what people see in imagery. The seller sees a monkey-like creature riding a mouse. The artist who created this seal design may have been mortified by this observation as he/she meant to show a witch riding a black cat. Dennison's designs were quite different from what had gone before during the period of ~1928-1931. This boxed set is a good example of the type of design Dennison produced during that interval.
Mark B. Ledenbach's vintage Halloween collectibles blog.