According to the “Petit Larousse Illustré,” un mouton à cinq pattes (literally, “a five-legged sheep”) means “an extremely rare phenomenon, thing or person.” The Collins-Robert French-English Dictionary translates it as “a rara avis or world’s wonder.”
Thanks for putting me on to this: I’d never heard the phrase until now, though I thought I was pretty well up in French.
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Dilige et quod vis fac