Cowsay

cowsay is a program that generates ASCII art pictures of a cow with a message. It can also generate pictures using pre-made images of other animals, such as Tux the Penguin, the Linux mascot. It is written in Perl. There is also a related program called cowthink, with cows with thought bubbles rather than speech bubbles. files for cowsay exist which are able to produce different variants of "cows", with different kinds of "eyes", and so forth. It is sometimes used on IRC, desktop screenshots, and in software documentation. It is more or less a joke within hacker culture, but has been around long enough that its use is rather widespread. In 2007, it was highlighted as a Debian package of the day.

Example
The Unix command fortune can also be piped into the cowsay command:

Using the parameter -f followed by tux, one can replace the cow with other beings, such as Tux, the Linux mascot:

Using the parameter -l shows all available cow files: