Ratatöskr, in Norse mythology is a squirrel that belongs to the creatures of the world tree Yggdrasil. He transmits messages between the eagle at the top of the tree and the serpent dragon Nidhögg at the bottom of the world tree that constantly insult each other.
The name "Ratatoskr" contains two elements: -rata- and -toskr. The element toskr is usually considered to mean as much as "fang". According to Guðbrandur Vigfússon's theory, -rati means "traveler," and the name of the legendary auger Rati may be derived from the same term. According to Vigfússon, Ratatosk means as much as "fang-traveler" or "fang-climber".
According to Sophus Bugge, Ratatosk may be a borrowing from Old English meaning "Rat-fang" or "Rat-tooth," but his theory does not look plausible in light of the fact that the element -toskr does not appear anywhere else in Old Norse. Bugge proposed that the -toskr element is a transformation of the Old English word tūsc (Old Norse - tusk), and the -rata element refers to the Old English word ræt ("rat").
Anna Kozlowska-Ryś and Leszek Ryś translate Ratatosk from Old Icelandic as "biting tooth".
Rudolf Simek believes that "the squirrel probably represents only an embellishment of the overall mythological picture of the World Tree in the Speeches of the High.
Hilda Ellis-Davidson, describing Yggdrasil, states that some suggest that the squirrel gnaws at it, maintaining the cycle of destruction and new growth that the Tree symbolizes.
John Lindow points out that Yggdrasil is described as rotting on one side, being gnawed on by Nidhögg, and the creatures that inhabit it swearing at each other in nasty words. As Lindow notes, "In the sagas the person who incites or sustains enmity by transmitting scoldings and threats between the warring parties is rarely of high social status, hence in the myth also the role seems to have gone to a creature not particularly significant."
Richard W. Thorington Jr. and Katie Ferrell suggest that "The Ratatosk may have as its prototype the European squirrel (Sciurus vulgaris), which when threatened makes a sound similar to a grunt. It doesn't take a particularly vivid imagination to think that a squirrel is scolding you."