After having forgotten this for
a long time, I recently learned from a reviewer, Andy Philpott, that this fable appeared in the 1974
novel "A Mote In God's Eye" by Larry Niven and Jerry
Pournelle (chapter 58).
The final possibility, "The horse may learn to sing," has become a catch phrase in our household. Either my wife or I may say it to the other when seeing some activity whose purpose seems unlikely to ever be achieved no matter how desireable. Whether it is meant as encouragement or ridicule is never quite clear. However, the ambiguity itself tends to keep the household happy.