The squirrel that carries insults up and down the World Tree
The squirrel that runs up and down the trunk of the World Tree Yggdrasil, serving as messenger between the eagle at the treetop and the venom-dragon Níðhöggr at the root. But what it carries is no goodwill — it bears slander back and forth, embellished, between the two, fanning ever hotter the enmity above and below. It is an odd, vivid presence in Norse myth: the most restless mouth on the tree of the nine worlds.
The eagle roosts at the very top of the World Tree, the venom-dragon Níðhöggr lies at its deepest root, and the whole vast tree stands between them, with no dealings of their own. Yet Ratatoskr darts back and forth in between, carrying each side’s abuse, embellished, to the other, delighting in sustaining this quarrel across the nine worlds — as though its whole reason for being were to keep the World Tree from ever knowing peace.
Below are how modern works borrow or reinterpret this name or story — not the original material. The two differ, so keep them apart.