As colony creatures, Pandronians existed as cooperative beings made up of discrete organisms. Consequently, the concept of an individual held a unique meaning for Pandronians, who referred to the colony as "this one" instead of the more expected "I" used by most humanoid species. The separate organisms that constituted a unique Pandronian may have also, over the course of time, "disassembled" and reassembled with other creatures creating a new colony and effectively terminating the existence of the previous unique Pandronian.
The individual organisms of a Pandronian were autonomous to a large extent. All parts were mobile on their own, through both bipedal locomotion and a form of levitation which has not been fully explained. Pandronians were typically of comparable height to Humans and Vulcans. They had green skin with tufts of red hair, large eyes, and a slightly down-turned mouth. Young Pandronians were referred to as "egglings", suggesting that they were laid in eggs, not born live. (TAS: "Bem")
Pandronians were first named "Schlossers," then "Shlossers," prior to the advent of their eventual name. They were initially conceived as a newly-discovered race from a part of the galaxy that had only recently been explored. (The Trek 25th Anniversary Celebration, pp. 51 & 52)
The Pandronians' appearance was at first far different from what it ultimately became. In an initial outline, a member of the species was described as a "bright blue-green creature like a giant tribble with arms and legs." The alien's detachable limbs were frequently referred to as "graspers and walkers," however. (The Trek 25th Anniversary Celebration) Additionally, the aliens were originally conceived as essentially consisting of two main parts. Writer David Gerrold recalled, "Actually, the original conception was that [it] would be two creatures in one, two species that lived in symbiotic partnership." ("Bem" audio commentary)
The alien species was initially to have been created for Star Trek: The Original Series. David Gerrold recounted, "I'd said, 'Here's how I think we can do the costume,' and I'd done a sketch."  His idea of using costuming to depict the alien was inspired by the fact that, as a student of the theater arts department at Cal State University at Northridge, he had recently taken part in an elaborately-costumed production of Winnie the Pooh in which he had played a rabbit. "It made me think what we could do with costumes to be outlandish," Gerrold explained. "And so I thought if we got two short people or two little people, one to play the bottom half of the creature and the other to play the top half, we could have some fun here." ("Bem" audio commentary) He elaborated, "[The] two-part creature [...] would be played by a very sturdy, short person who would be carrying a very short, shorter person on his back." The alien's creation was made easier, so thought Gerrold, by the fact that it was, instead, ultimately done for Star Trek: The Animated Series, since animating the alien's many parts was a lot more manageable than having to worry about building a costume and finding suitable actors. ("Drawn to the Final Frontier - The Making of Star Trek: The Animated Series", TAS DVD)
There was a similar species to the Pandronians, the Zykarian, in the 1979-1980 Buck Rogers TV series episode "Journey to Oasis". The Zykarian character, Ambassador Arom Duvoe, was played by actor Mark Lenard.