Ratatosk

The furry, chattering ratatosks are the dominant race of Yggdrasil, the World Ash. They are the consummate messengers of the planes, able to sure-footedly climb from the Gray Waste to Elysium and to all the worlds and half-worlds touched by the Really Big Tree, and glide down again with the furry membranes that stretch between their limbs. They consider the Tree to be their mother and chief provider, although most of the luxuries they gain - exotic foods, useful weapons,  and interesting trinkets to line their nests - are gained in exchange for delivering messages for bodies and souls throughout the multiverse.

They look like nothing so much as giant squirrels.