Raven
The raven is one of the most important birds in Celtic mythology (and there are many important types of birds). The raven is a symbol of death and war, but also of the otherworld and its eternal pleasures. In Ireland, the raven is especially associated with Badb, whose name means "raven" and is an aspect of the Triple Goddess, Morrigan or the Morrigue. Her sisters Macha and Neman also appear in the forms of ravens on the battlefield.
In Welsh myth, the raven is associated with Bran (Bendigedfran ap Llyr), the Fisher King. He is lord of the otherworld, the feasting head who owned the cauldron of rebirth.
The raven is also associated with Owain ap Urien, one of Arthur's greatest knights. While Owain was a historical figure, he became mythologized, to the point that his mother is sometimes listed as the goddess Modron, and later Morgan le Fay, who is, of course, the same raven goddess Morrigan.
The raven is a symbol of death, but the Celts, believing in both a happy otherworld and reincarnation, did not fear death, but welcomed it as part of the cycle, over which the raven rules.