This is one of some hundred of Aesop's fables that were not published in English translation until the Penguin edition of 1998. The editors of that edition feel that these omissions were due to prudery; it seems to me as likely that it is some of them were due either to scepticism about the authenticity of the fable itself, or to the (generally correct) feeling that these fables are not of much interest, or both. For what it is worth, here is "The Beaver", in the Latin translation of the Heinsius edition of 1657, p. 32:
Fiber animal est quadrupes, in stagnis plerumque vitam degens, cujus pendenda dicunt usui esse medicis. Hic igitur cum ab hominibus persequentibus jam capiendus sit, cognito cujus gratia petatur, abscissa sua pudenda projicit persequentibus, & sic salutem consequitur.
Affabulatio.
Fabula significat, eo modo homines prudentes pro sua salute nullum habere respectum pecuniarum.
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