“Are there any mythical beasts which aren't simple pastiches of nature? Centaurs, minotaurs, unicorns, griffons, chimeras, sphinxes, manticores, and the like don't speak well for the human imagination. None is as novel as a kangaroo or starfish.”

Gregory S. Paul (2010) The Princeton Field Guide to Dinosaurs, Princeton University Press, p. 9
The Princeton Field Guide to Dinosaurs

Source: Howl and Other Poems

Variant: It's like learning to ride a unicorn. You never forget.
Source: Artemis Fowl

Source: Storm Front

This phrase seems to have been first mentioned in Manual of a Perfect Atheist by Mexican writer Eduardo Garcia Del Rio, in 1989, without indicating any original source, which does make this quote unreliable. The quote has been widely circulated by atheists to try to prove that Chaplin was also one of them. However, taking into account what Chaplin himself wrote in his autobiography, when he was 75 years old, and what his family members wrote about him, calling Chaplin an atheist seems untenable. (http://www.adherents.com/people/pc/Charlie_Chaplin.html) According to his son, Charles Chaplin, Jr., in his book "My Father, Charlie Chaplin", pages 239-240, Chaplin was not an atheist; he quotes him saying: "I'm not an atheist"… "I can remember him saying on more than one occasion. 'I'm definitely an agnostic. Some scientists say that if the world were to stop revolving we'd all disintegrate. But the world keeps on going. Something must be holding us all in place — some Supreme Force. But what it is I couldn't tell you.". See also pages 210-211 of the book.
Disputed

Source: Looking Backward, 2000-1887 http://www.gutenberg.org/dirs/etext96/lkbak10.txt (1888), Ch. 1.

1840s, Letters from New York (1843)
Source: Letters from New York http://www.bartleby.com/66/59/12260.html, vol. 1, letter 34