Marbleworld
“The disc, being flat, has no real horizon. Any adventurous sailor who got funny ideas from staring at eggs and oranges for too long and set out for the antipodes soon learned that the reason why distant ships sometimes looked as though they were disappearing over the edge of the world was that they were disappearing over the edge of the world.” — Sir Terry Pratchett, The Light Fantastic
Great A'Tuin is the Giant Star Turtle (of the fictional species Chelys galactica) who travels through the Discworld universe's space, carrying four giant elephants (named Berilia, Tubul, Great T'Phon, and Jerakeen) who in turn carry the Discworld. The narration has described A'Tuin as "the only turtle ever to feature on the Hertzsprung–Russell diagram" -
wikipedia page for Discworld