As Hackett explains in his video, Clarus was designed by graphic designer Susan Kare in 1983. Kare created the various fonts for the Macintosh's user interface, and one font in particular had a unique set of glyphs and images to represent the alphabet, referred to as a dingbat font. The letter "z" was represented by a small creature that appeared to merge a dog together with a cow.
Clarus became so endearing to Apple engineers that the Dogcow appeared in various printing programs on Macintosh computers, indicating to users which orientation the paper would be in when it was printed. Eventually, Scott Zimmerman coined the term "Dogcow" in 1987 and Apple employee Mark Harlan named the character Clarus in Technote 31 in the Developer Technical Support documents, to clarify the small animal people were seeing in the printing software.
In the entry called "The Dogcow," Harlan stated, "Dogcows, by their nature, are not all dog, nor are they all cow, but they are a special genetic hybrid. They are rarely seen in the wild. Since dogcows are two dimensional, they will stand facing a viewer “on edge” to avoid being seen."
Hackett: "Apple was still performing well at this point, with the dark days of the mid-90s still several years off, and the company had a sense of humor about itself."Clarus the Dogcow rose to prominence in the early and mid 1990s, thanks to a few videos of the character in early versions of QuickTime. One video shows Clarus riding a merry-go-round, while another has the character spinning in various directions.
At this time, the appearance of Clarus as an Apple icon really began to take off, with the Dogcow showing up in official Apple documentation about rendering on-screen graphics, mousepads, t-shirts, a brand of beer called "Moof Bräu", and even in the Icon Garden on Apple's Infinite Loop campus.
As the 1990s ended, so did the height of Clarus the Dogcow. According to Hackett, "While it's hard to pin the Dogcow's decline directly on Steve Jobs, Clarus became harder and harder to spot after his return to Apple. The Icon Garden came down, and Mac OS X used a less-fun image on the Page Setup screen. While Clarus made a brief appearance with OS X’s Address Book, it was hardly a comeback. The party, as they say, was over."
Clarus has made occasional appearances in recent years, with WWDC developers last year receiving a Clarus the Dogcow pin as part of their swag bags.
Those interested in the history of Clarus the Dogcow should check out the 512 Pixels video and Hackett's History of Clarus the Dogcow.