The Betta fish

Many people haven’t seem to have caught the subtle joke with the Siamese fighting fish that is part of the default background. Another name for the siamese fighting fish is the Betta Splendens , also known as the "betta fish" or just "betta". Wikipedia states that betta is pronounced as the Greek, beta. Hence the  fish in the background of Windows 7 Beta.

Here is another image of the betta fish I found in thevista.ru