According to Wikipedia, several municipalities in France are named Faverolles (with a final S), and Faverolle (without the S) appears to be a minor stream only. Most likely the name of the place the chicken breed originated from did have a native S (not to be pronounced in French), and its presence at the end of the name has nothing to do with the English language adding a possessive S to the original French name (a feature that does not exist in the French language).

This chicken breed, called “race de poules domestiques” in French does come without doubt from a place called Faverolles; this is indicated in the French online free encyclopedia, in the section called “Autres”.
There are 2 such breeds and both names take an s : “Faverolles allemande” and “Faverolles française”.

The “Faverolles française”, also called “faverolles” originated in Faverolles, name of a village (population 900) and name also of the smallest administrative territory this village is found in (commune in French), this “commune” being in the “département” called “l’Eure-et-Loir” (near Paris). The “Faverolles allemande” was originally a sub-breed of the faverolles that has been raised in Germany since 1912.

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