Ratatoskr

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.
Attestations.
In the "Poetic Edda" poem "Grímnismál", the god Odin (disguised as "Grímnir") says that Ratatoskr runs up and down Yggdrasil bringing messages between the eagle perched atop it and Níðhöggr below it:
Ratatoskr is described in the "Prose Edda"s "Gylfaginning" chapter 16, in which High states that
An eagle sits at the top of the ash, and it has knowledge of many things. Between its eyes sits the hawk called Vedrfolnir [...]. The squirrel called Ratatosk [...] runs up and down the ash. He tells slanderous gossip, provoking the eagle and Nidhogg.