E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Rara Avis (Latin, a rare bird).
A phenomenon; a prodigy; a something quite out of the common course. Black swans are now familiar to us; they are natives of Australia, and have given its name to the Swan river. At one time a black swan was emphatically a rara avis.