b.white sock, a white portion on the leg of a horse, extending half-way up to the knee.
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the world > animals > mammals > group Ungulata (hoofed) > family Equidae (general equines) > colour or marking > [noun] > mark or streak > white mark or patch
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white sock1893
1893 M. H. HayesPoints of Horse (1897) xx. 222 ‘White stocking’..might be reserved for one that comes up as high as the knee or hock; while that of a ‘white sock’ might be used to signify the marking when it is shorter.