Does someone know the date (year) of the first record or sheet music release of the Shel Silverstein Unicorn song?

(I wrote a song about a different mythical beast not getting on the ark, and at the time I wrote it, I had never heard the Unicorn song. [The story and music are also quite different from Silverstein's.] I am curious as to whether the Irish Rovers' or other's recordings of Shel's song were out and were widely known at the time I made up my story and song.

Sonja