Jinx is an Earth-sized moon of an extra-large gas giant planet, Binary, orbiting the star Sirius A in several of Larry Niven's Known Space science fiction stories. Niven never actually sets a story on Jinx, but he gives us a physical description several times.
Jinx is actually a bit bigger than the Earth, as it has a gravity about 1.5 times that of the Earth.
More interestingly, Jinx takes the form of a prolate spheroid (tending towards the shape of a rugby ball or American football). It was pulled into this shape during its formation by tidal forces from binary, then stayed that way when it cooled1.
Near Jinx's equator, the atmosphere is a Venus-like soup (the oceans are concentrated there, too, but we'll get back to that). The two poles (known as the East End and the West End stick up out of the atmosphere. The midlatitude zones, however, are just right for human habitation, if you can stand the world's gravity.
Jinx was one of the first worlds detected by automated probes sent from Earth, and one of the first worlds colonized by humans (using slowships). By the time of Ringworld, Jinx was one of the primary centers of human civilization.
Around the shores of the equatorial sea crawl huge sluglike creatures known as bandersnatchi. These were put there by the psychic race of Thrintun, aka "Slavers", a billion years before the arrival of humans. Bandersnatchi were the principal food source of the Slavers before they were annihilated during the uprising of their technological slave race, the tnuctipun (who were also annihilated along with the rest of galactic civilization).
Bandersnatchi are intelligent; their initial commmunication with humans involved grazing to form huge patterns on the ground in Slaver writing. Although it is not explicitly stated, communication with bandersnatchi was probably eventually done via telepathy.
The bandersnatchi have an unusual economic agreement with the human residents of Jinx: money in return for the right to hunt bandersnatchi, a prestigious sport among the wealthy of Known Space.
1Unfortunately, this is physically impossible. The real Jinx would have sprung back into a spherical shape by the force of its own gravity.