A Cow. A Trampoline. Together, they fight crime. Copyright © 2003 by Jamie Zawinski. Moo. Permission to use, copy, modify, distribute, and sell this software and its documentation for any purpose is hereby granted without fee, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation. Moo. No representations are made about the suitability of this soft‐ware for any purpose. It is provided "as is" without express or implied warranty. Moo.

This is normally part of xscreensaver which means you have to put a load of other junk on your computer to get it. Here is a .deb package with just Bouncing Cow in it. After installing it Bouncing Cow will be added to the list of available ones.

The number of cows and the speed they bounce can be changed by editting the file /usr/share/xscreensaver/config/bouncingcow.xml.