They descended from a race of saber-toothed turtles who evolved to walk upright. They averaged in height from about 1.75 meters to 2 meters. They only had one sex and reproduced by laying eggs. (TOS reference: The Making of Star Trek: The Motion Picture) Chelon performed male or female gender roles at different points in their lives. Many Chelon often took a permanent gender role of male or female, however traditionalists in their society refused to take on such gender roles. (ENT - Rise of the Federation novel: Tower of Babel)
Though they lacked agility, they were a hardy race, and their shells gave them excellent protection. As they evolved in the Rigel system, they were highly resistant to ultraviolet radiation, with a lesser resistance to other kinds of radiation. As an aquatic species that dwelled in swamps, Chelarians were skilled swimmers. They preferred the warm and humid weather of Rigel III and found most other worlds uncomfortable.
A Chelarian's fangs gave it a powerful biting weapon (Decipher RPG module: Worlds). Chelons, during times of stress or combat, emitted a deadly contact toxin through their skin, which could be transmitted via a claw strike. (ST - Destiny novel: Mere Mortals)
Rigellians had attendants who served, fed, and cared for them. This gave the attendants the real power in Rigellian society. When worn, armor served a mainly ceremonial function. (TOS reference: The Making of Star Trek: The Motion Picture)
Despite their natural weapons, armor and size, Chelarians were actually relatively peaceful. Modern, civilized Chelarians had little skill with their bite attack. Instead, some trained to use it in combat.
The Chelarian homeworld was Rigel III, though many also lived on Rigel II. Here, they had a number of steadings in the swamps, where they grew more difficult crops than on the neighboring plantations. They worked as middle-men between the local Orions and Vulcanoid Rigelians. (Decipher RPG module: Worlds).
Older sources called the Chelarians "Rigellians", though that name fell into disuse to avoid confusion with other races going by the names "Rigellian" and "Rigelian".
The Chelarians became members of the Federation in 2275 (Decipher RPG module: Worlds). The Federation's 23rd century Vanguard operation was represented by a Chelon, Ambassador Jetanien. (VAN novel: Harbinger) Later, in 2379, a Chelon served as a biologist aboard the USS Titan. (TTN novel: Taking Wing)
In some novels these Rigellians are presumed to come from the same solar system – Rigel – as the Rigelians; their specific species being "Chelon," or "Rigellian Chelon." When Mego produced a 3.75" Saurian action figure for its ST:TMP line, the company inadvertently called it a "Rigellian". In the Last Unicorn Games and Decipher roleplaying games, they are called "Chelarians" and hail from Rigel III (Chelar).
Details about this species, including their name, comes from notes created by Fred Phillips and Robert Fletcher for Star Trek: The Motion Picture and can be found in The Making of Star Trek: The Motion Picture.