In the
LOGO educational programming language, the Turtle was your
cursor. You were supposed to
imagine a turtle with a
pen in its
stomach that would follow LOGO
commands on your behalf,
drawing pictures with its pen. Some
implementations helpfully drew a picture of a turtle to help you imagine.
In Terry Pratchett's Discworld universe, the turtle is what holds the four elephants, which in turn hold the Discworld. The turtle is also the form of the god Om when it is... uh... small, in the book "Small Gods".