To celebrate Apple's 35th anniversary, NetworkWorld has published a list of its favorite Apple based April Fools' Day jokes over the years.
While Apple has in no way embraced April Fools' Day with the vigor of a Google, using the latter's search engine for a few hours does turn up a bushel of Apple-related April Fools' Day foolishness.
April 1, 1989: Clarus the Dogcow The oldest I could find and one of the best, this submission was fashioned as the now legendary TechNote No. 31 by Mark "The Red" Harlan and delivered to the Apple developer community through its monthly bulletin. Purporting to tell the story behind the dog-and-cow-like image used by Apple for various purposes since 1983, it begins:
Recently we've been getting a lot of questions about the dogcow in (Developer Technical Support). The purpose of this Technical Note is to clear up any confusion that you may have, and more importantly, to get you to quit bugging me about it.
What is a Dogcow? I wish I had a nickel for every time I've heard this question--by now I'd have enough money to buy a "Changing the world, one person at a time" bumper sticker. Dogcows, by their nature, are not all dog, nor are they all cow, but they are a special genetic hyb.rid. They are rarely seen in the wild. Since dogcows are two dimensional, they will stand facing a viewer "on edge" to avoid being seen.
Harlan went on to reveal for the first time that Apple's Dogcow has a name, Clarus. Today, Clarus has her own online "museum," which you may browse at your leisure