Ratatoskr
In Norse mythology, Ratatoskr is a squirrel that lives on the tree Yggdrasil. Ratatoskr carries messages between the eagle at the top of  Yggdrasil, and the serpent Níðhöggr, at the bottom. Ratatoskr occurs both in the Poetic Edda and the Prose Edda. The Poetic Edda is a collection of earlier stories. It was written in the 13th century. The Prose Edda was also written in the 13th century.