Dobbin

wiktionary

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From Dobbin, a common name given to workhorses.

etymonline

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dobbin (n.)

common name of a work-horse or farm horse, 1596 (in "Merchant of Venice"), probably from diminutive form of Dob (early 13c.), the common Middle English familiar form of the masc. proper name Robin or Robert; the personal name being applied to a horse.