Kelly
As Martin has pointed out there are a number of Cowans in Burke's General Armory. The Harleian Society's Grantees of Arms lists two and the Scottish Ordinary, vol I, lists seven.
My best guess at this stage is that the Arms supplied by heraldy bucket shops are actually those granted in 1883 to Lt-Col Phineas Cowan of Lancaster Gate, London (1832-1899).
Yours aye
Stephen