Heiðrún

In Norse mythology, Heidrun (commonly pronounced as HI-TH-RRUN) is the great goat of Valhala. The Einherjar, warriors who died in battle for Odin, get their mead from this animal. She lives at the top of Valhalla. Heidrun eats leaves from the trees on the top the fortress. 
"Poetic Edda".
In the "Poetic Edda" Heiðrún is mentioned twice. She is described in the "Grímnismál".
Portrayals.
In most portraits that show the goat, she is shown as a very large creature standing on top of the roofs of Valhalla. She is seen in a way similar to Ratatoskr, the gossip loving squirrel that tends the world tree. Because of this, some historians think that Heidrun would eat the leaves from the upper branchs of Yggdrasil that are pushed to Valhalla. This may mean that Asgard is also up there. The Norse were very vague about where they thought the realms were on the tree.