A Megarite passed through a Starfleet Headquarters air tram station in San Francisco on stardate 7410.2, moments before Admiral James T. Kirk departed from an air tram. (Star Trek: The Motion Picture)
The name "Megarite" was introduced to the world nearly a full six months before the release of The Motion Picture. During her visit to the set of The Motion Picture, Susan Sackett noted many "curious creatures", including the "hairy Megarites", which she contrasted with the "hairless Deltans." (Starlog #23, June 1979, p. 31.)
The Megarites were designed by Fred Phillips and Robert Fletcher for The Motion Picture. With the approval of Gene Roddenberry, the Megarites, as well as numerous other new species in the film, were named by Fletcher and provided a backstory by him. The following is a brief description, in Fletcher's own words:
"MEGARITES – Quite humanoid, but body has similarity to rhino hide. Four lips, with straning mechanics similar to baleen of whale, in mouth of whales. Live by marine life found in waters of their planet, difficult to live away, have to provide artificial means of keeping alive in other environment. "Federation scientists have found way by intravenous injection of proper nutrients capable of sustaining them for a week. Poetic people, communicate with songlike sounds, musical poems in own language, through four lips. Most of planet made of jade – houses, artifacts, etc., carved from jade. Much of clothing comes from sea animals. Costume: sleeve, cast from plastic, simulates graphite steel. Traditional on planet to wear many ornaments of graphite steel or jade. 'Jade' plaque on costume collar made of fabric and plastic, looks like jade. Hood of fabric lined with sheet foam rubber to give body sculptured effect. Costume of indigo blue wool, from studio warehouse." (The Making of Star Trek: The Motion Picture, p. 132; Starlog #33, April 1980, p. 71)
"Who ever coined the phrase 'hide as thick as a rhino's,' must have met a Megarite. All Megarites have very tough, thick skin which does resemble that of a rhino. But they are a gentle people who communicate in a lyric type of poetry which their four lips alone in the galaxy are able to produce. Their musical poetry is treasured throughout the universe but due to the fact that they find it not only unpleasant but extremely difficult to leave their home planet, few except their diplomats, ever do. Because of this fact, until recent developments by Federations [sic] scientists, only those individuals who could travel to their planet could experience the Megarites' beautiful music. Federation scientists developed intraveneous injections of the proper nutrients for Megarites which allow any Megarite who wishes to do so to travel freely about the galaxy. On their home planet they remain close to the water and survive off plankton which is found there. The sole export besides their music is jade which is in such abundance that they make not only adornments but their homes, tools and the majority of their artifacts from this mineral." 
In an interview appearing in the March 1980 edition of Fantastic Films, part two of "The Star Trek Costumes", Fletcher revealed more information about the Megarites and the design of their costumes:
"These are the Megarites. I call them the jade people because their planet has jade in such abundance that they make everything out of it. That even includes their houses. The bracelets are cast of plastic and made to look like graphite steel. They also have a great deal of graphite steel on their planet so they wear a great many ornaments made out of this material too. The jade plaque on his collar was made of a felt and resin combination. Notice the hood. The hood of the costume was is lined with a sheet of foam rubber to give added body to the costume and to create a more sculpted look. The hood then fits directly onto the collar. The particular costume is made of an indigo blue wool. The loose folds and the very rounded head help give a very otherworldly look to the creature."
In an article appearing in the January 2002 edition of Star Trek: The Magazine Volume 2, Issue 9, entitled "Who is that Alien?", the following information about the Megarites, as taken from Fletcher's original background notes, added:
"In their natural environment, the Megarites lived under water, but the Federation developed a system of injections that allowed them to survive on land for about a week. Interestingly, most of their planet was made from jade."
According to the novel Ex Machina, the USS Enterprise had a female Megarite crewmember named Spring Rain on Still Water.