What is the name of the squirrel that runs up and down Yggdrasil?

The squirrel that runs up and down Yggdrasil is Ratatoskr.

In Norse mythology, Ratatoskr travels along the world tree, carrying messages between the eagle perched in its crown and Níðhöggr, the dragon-serpent beneath one of Yggdrasil’s roots. The squirrel is therefore part of the tree’s living, interconnected world.

The medieval poem Grímnismál names Ratatoskr while describing Yggdrasil’s creatures. Snorri Sturluson’s Prose Edda expands the image, saying that the squirrel carries slanderous gossip and helps provoke hostility between the eagle and Níðhöggr.

A common mix-up is Veðrfölnir, the hawk said to sit between the eagle’s eyes. Níðhöggr is the serpent below the tree, not the squirrel. Ratatoskr’s name is often interpreted as meaning “drill-tooth” or “bore-tooth,” though its exact etymology is discussed by scholars.