The Wikipedia article about dogcow states that someone else made it:

> In 1983, the dog icon had been created by Susan Kare as the glyph for "z", as part of the Cairo font. Later, when designing the classic Mac OS "Page Setup" print dialog box, an example image was required to demonstrate the orientation and color of the paper.[2] HCI engineer Annette Wagner made the decision to use that dog as a starting point, editing it and creating a larger version with spots to be more suitable for demonstrating various printing options. The new dog graphic had a more bovine look.

It claims that Susan Kare made the dog icon but Annette Wagner made dogcow based on it.