Brock

... is an Old English word for the badger, borrowed from the Celtic languages.

The name 'badger' was first used in the 16th century. Wikipedia suggests that it "presumably [refers] to the white mark borne like a badge on its forehead", but gives other possible derivations: it may be a variation on an old French word meaning 'striped' or 'piebald', or (from arcane Germanic roots) a reference to its digging of setts.