Short answers, pulled from the story.
Ratatoskr is a squirrel in Norse mythology who runs up and down the world tree Yggdrasil carrying messages between the eagle at its crown and the dragon Níðhöggr beneath its roots. According to the Prose Edda, Ratatoskr tells slanderous gossip that provokes both creatures.
Most modern scholars translate Ratatoskr as "drill-tooth" or "bore-tooth." Scholars including Jesse Byock, Andy Orchard, and Rudolf Simek favor "drill-tooth," while John Lindow prefers "bore-tooth."
Ratatoskr is attested in the Poetic Edda, compiled in the 13th century from earlier sources, and in the Prose Edda, written in the 13th century by Snorri Sturluson. The Poetic Edda poem Grímnismál contains the earliest direct description, spoken by Odin in disguise as Grímnir.
John Lindow suggests that in Norse sagas, a person who keeps feuds alive by carrying malicious words between rivals is seldom of high status, which may explain why this role falls to a small, relatively insignificant animal. Richard W. Thorington Jr. and Katie Ferrell propose the myth derived from the scolding alarm calls of European tree squirrels, Sciurus vulgaris.
Ratatoskr appears as a prominent side character in the 2022 video game God of War Ragnarök. A ghostly version of the same figure also appears in the preceding title God of War as an ally of the character Atreus.
Songs about Ratatoskr appear on Ýdalir, the sixth album by the Icelandic viking metal band Skálmöld, and on Fimbulvinter, the third album by the Swedish viking metal band Brothers of Metal.