In Norse mythology, Ratatoskr is a squirrel that lives on the tree Yggdrasil. Ratatosk 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.