The Swan is a potent symbol that is present through multiple mythographies. The appearance of the Swan is usually accompanied by a transformation of some kind and tension between opposites - example being that in Leda and the Swan, Zeus is the Swan and representative of the Sun while Leda symbolizes the Moon. The alchemy of opposites is also implied in the poem as the White Swan denotes the whitening process, a step along the journey to the quintessence in the alchemical marriage and a process with which Yeats was certainly familiar.
Blackening - Black Crow, Raven, Toad, Massa Confusa.
Whitening - White Swan, White Eagle, skeleton.
Greening - Green Lion.
Rapid cycling through iridescent colours - Peacock's Tail.
White Stone - Unicorn.
Reddening - Pelican feeding young with its own blood, cockerel.
Final transmutation - Phoenix reborn from the fire.