You can download a copy of Technical Note TN 1031 here (PDF). It's one of the few dogcow-related tech notes that still exist on the Apple server. For whatever reason, many of the original dogcow documents have been removed from developer.apple.com--you get redirected to the main page. MacFreek.nl has archived the text from Technical Note TN 31, the quintessential dogcow technote. This document answers the questions: "What is a Dogcow?", "But What Does This Have to do With the Macintosh?", and "Okay, So How Do I Draw a Dogcow?" It also points out that there are no known "cliff-dwelling dogcows" left in the wild.
In the end, Clarus the DogCow is a reminder that Apple was once smaller than it is now and that its employees actually had a sense of humor. You can find an excellent overview of the entire phenomenon at StoryBytes.
 By definition, there are the kind of people who divide people into two kinds of people and there are the kind who do not.