Yggdrasil - "noblest of trees" - a immense holy ash tree that connects all the nine worlds of the Norse mythology.
Huginn and Muninn - in old norse "thought" and "memory". Two ravens flying around the world and bringing news to their master Odin.
Dáinn, Dvalinn, Duneyrr and Duraþrór - four stags (or harts) living in the treetop of Yggdrasil and gnawing on its branches. Their symbolic meaning is still unknown due to trouble with translating the original source but it's said they might be connected with the four elements, the four seasons or the phases of the moon.
Eagle and hawk Veðrfölnir - in the poem, the hawk is sitting between the eyes of the eagle. The two are an inseparable pair where hawk represents the power and eagle the wisdom.
Ratatosk - a squirrel messenger that runs up and down Yggdrasil to tell slanderous gossip and envious words, provoking the eagle and Nidhogg.
Hati and Sköll - sons of Fenrir Hróðvitnir. The two chase respectively chariots containing the sun and the moon. At Ragnarök, both Sköll and Hati will succeed in their quests catching and devouring them.
Fenrir - a monstrous wolf, one of the children of the giant Loki, foretold to kill the god Odin during the Ragnarök. Till then he's bound with a mythical rope Gleipnir on a an island Lyngvi (a place overgrown with heather).
Gleipnir - a rope or a chain created by dwarves for the very purpose of capturing Fenrir. It was made of the sound of a cat's footfall, the beard of a woman, the roots of a mountain, the sinews of a bear, the breath of a fish and the spittle of a bird.
Níðhöggr - a serpent, wyrm, dragon that lives under the Yggdrasil and gnaws on its roots.
Mjǫlnir - in this tattoo hidden in the general shape of the design - hammer of the thunder god Thor. The most powerful weapon in the world capable of leveling mountains.