cowsay is a program which generates
ASCII 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. .cow 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.