Wikibooks, withers derives from an adjective and as such is plural in form only, not in meaning.
Etymonline has it as plural, 'the withers are the parts of the animal that oppose the load.' Hmm, if 'withers' refers to one object, how can 'withers' be defined as 'parts of the animal'? Etymonline, are you sure? Dictionary.com says 'wither' is used with a plural verb. Oxford paperback calls 'withers' a plural noun, but doesn't provide an example for subject-verb agreement.
My advice, choose the dictionary you like and follow their advice. As for me, 'the withers is a part of the animal' is the way I would say it, be it deemed correct or not.
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