In 2001 you can enable an easteregg where you can drive around and shoot down flying cows: Open the Edit Projekt dialog and insert Holy Cow! as Project Name, this will unlock the Game Demo button.

In 03 there are some cows at the end of the first part of the demo.

In 05 you can enable an easteregg that replaces some of the the Arolphins (flying dolphins) with Cowlfins (flying cows) at the end of the Demo (credits screen):

3DMark05 EasterEgg - Flying Cows

Thanks to Addicted Tweaker