Diabolus

The Diaboli (singular: Diabolus) are a sapient spacefaring species and masters of a unified interstellar nation encompassing hundreds of planets in the year 4699 AD. Their nearest planet is located 1500 light-years from Earth.

Biology
Diaboli are larger and more massive than Humans. They walk on four short legs and have two arms with flexible digits. They have hairless wrinkled skin, large-pupilled eyes and short horns. They are herbivorous ruminants and have two mouths, consisting of a primary mouth for eating and a smaller one that can be used for speaking.

The Diaboli breathe sulphurous anoxic air. Their metabolism is based on reducing sulphur into hydrogen sulphide. They can't live in oxygen atmospheres, and carbon dioxide is poisonous to them.

Although they are capable of speaking with their secondary mouths, the Diaboli prefer to communicate among themselves by ultrasonic waves produced by rapidly vibrating the skin between their horns using specialized muscles. While their hearing range extends into the ultrasonic, there are sounds in the audible range that Humans can perceive that the Diaboli cannot.

Culture and society
The Diaboli are a unified people who venerate their own homeworld and consider the separation of a single species into opposing nations to be unthinkable. The idea of one faction declaring independence and/or waging war upon another is akin to cannibalism to them, enough to make them feel sick. They don't wear clothes.

Appearances

 * "In a Good Cause", by Isaac Asimov (1951)