The dun cow of Dunsmore heath was a savage beast slain by Sir Guy, Earl of Warwick. A huge tusk, probably that of an elephant, is still shown at Harwich Castle as one of the horns of the dun-cow. (See Guy.)
The fable is that this cow belonged to a giant, and was kept on Mitchell Fold (middle fold), Shropshire. Its milk was inexhaustible; but one day an old woman who had filled her pail, wanted to fill her sieve also. This so enraged the cow, that she broke loose from the fold and wandered to Dunsmore heath, where she was slain by Guy of Warwick.
Isaac Taylor, in his Words and Places (p. 269), says the dun cow is a corruption of the Dena Gau or Danish settlement in the neighbourhood of Warwick. Gau, in German, means region, country. If this explanation is correct, the great achievement of Guy was a victory over the Danes, and taking from them their settlement near Warwick.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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