The Dogcow is a now-infamous creature created by Apple's Developer Technical Support engineers. These creatures, as the name implies, are part dog and part cow, a genetic hybrid of the two. The dogcow's characteristic sound is the "Moof!" (or the "!fooM"). The particular dogcow you will see the most in Apple software and websites has a name - Clarus (no, not Claris, as in the company formerly associated with apple... I said Clarus, with a u).
Many dogcows are two dimensional and stand edgewise to avoid being seen. Not a creature of the wild, you will often find the dogcow in your very own lawn (but being a stupid creature, it will usually get mowed by you). However, there ARE known reports of "psychic" dogcows brainwashing humans, but those are unconfirmed. Dogcows look like dogs with solid legs and a flat, cow-like face, and spots all over, also like cows, resembling both and neither at the same time.
The dogcow first appeared in the Cairo font (designed by Susan Kare) but as that font was eventually dropped, it moved to other places in the Macintosh. The term "dogcow" was coined by Scott "ZZ" Zimmerman on October 15, 1987 (though some say Ginger Jernigan created the term a few days before). Two years later, the infamous Tech Note #31 was circulated, showing off the dogcow to everyone (it was intended as an April's Fool dare, though it really wasn't on quite the right day). In the months that followed, the Dogcow caught on and appeared on t-shirts and pins and other paraphernalia.
Dogcows have been known to appear in AppleDTS Tech Note #31, Develop Issues 17 and 18, and Tech Notes PR 510, PT 35, and TN 1019. Oh, yeah, the dogcow and "Moof!" are trademarks of Apple.
A dogcow is what I want to be.
Pictured in dialogs,
Running through the weeds,
In and out of advertisements,
Loving my naughty deeds.
Feeling in black and white.
Over the edge of cliffs,
Out with the tide in the sea.
Living life to the fullest,
Sweet survival in 2 D.
Nearly everything in this writeup, including the poem above, has the AppleDTS Dogcow site as its source. The URL is: http://developer.apple.com/products/techsupport/dogcow/ - read it; there's a lot more on the site that isn't here