The equinilus is found in some medieval encyclopedias. It is a large and ferocious monster that lives in the Nile River (equinilus is literally "Nile horse"). It is always eager to kill humans, and tears ships apart. It has a very thick skin and can only be captured with iron chains and only killed with iron hammers.

There are similarities between the equinilus and another monster of the Nile River, the hippopotamus. Their names have a similar meaning, they both live in the Nile, and both are said to have thick and impenetrable skin.