The dogcow, also known as Clarus the Dogcow, is a bitmapped image first introduced by Apple. It is the shape of a dog, originally created in 1983 as part of the Cairo font by Susan Kare as the glyph for “z.” That image was later chosen for the Mac OS Page Setup dialog box, though it needed to be slightly redrawn because the original Cairo dog did not proportionally fit the Page Setup dialog box. This modified version became the image famously known as the dogcow.
The term “dogcow” was first coined by either Scott Zimmerman or Ginger Jernigan. Mark “The Red” Harlan named the dogcow “Clarus” as a joking reference to Claris, Apple’s office software unit at the time. The sound she makes is “Moof!”
The image of the dogcow was used to show the orientation and color of the paper in Mac OS page setup dialog boxes. HCI engineer Annette Wagner made the decision to use the dog from the Cairo font as a starting point for the page graphic. Wagner edited the original font and created a larger version with spots more suitable for demonstrating various printing options. The new dog graphic had a more bovine look, making it arguably less clear as to what animal it was intended to be, and after the print dialog was released the name “dogcow” came into use.
Dogcow also made appearances in Apple Developer CDs. In early volumes, a section was known as "Moof!".
Excerpts from Technote 31 
Apple Technote 31 includes a description of the dogcow.
- There is a life-size picture of a dogcow conveniently located in the Finder. Look under “Page Setup...” Now look under “Options.” Voila, there is the dogcow in all its raging glory. Like any talented dog, it can do flips. Like any talented cow, it can do precision bitmap alignment.
- Somewhere along the line I baptized the dogcow “Clarus.” Of course she’s a female, as are all cows; males would be referred to as dogbulls, but none exist because there are already bulldogs, and God doesn’t like to have naming problems. (from History of the Dogcow, part II)
See also 
- Archive of a page of official Apple information on the dogcow
- Technical Note 1031 on Apple’s website
- Photograph of the dogcow in the Apple Icon Garden
- Jory's Dogcow Shrine
- A dogcow history around a museum of all its incarnation