"Moof" is what Clarus the Dogcow says. Clarus is somewhat of an Apple developer mascot. As the Apple Turns tells the tale of a Clarus-modded iBook that received a smile of recognition from Apple founder/CEO Steve Jobs at a WWDC.
But I digress. I'm guessing that this little "Moof" Easter egg was being used by developers as a placeholder or something and was accidentally left in the Mac OS. It sure was fun to come across it, though!
Let me assure you that this is a real Easter egg, not something I made up; nor is there anything at all on the network named "Moof". Here are the circumstances involved so you can try to reproduce it on your own systems: Screenshot taken on Mac OS X 10.2.8 running a Classic app. This is a standard Save dialog in Navigation Services-enabled Mac OS 9 apps. The network volume is a shared folder on a Windows 2000 Server with AppleTalk sharing enabled.