Chameloids were a species of shapeshifters.
Prior to 2293, James T. Kirk had heard about chameloids, but considered them a myth. That year, Kirk and Leonard McCoy met a chameloid named Martia on Rura Penthe. (Star Trek VI: The Undiscovered Country)
In the script for Star Trek VI, Martia was said to be from Arc, a location where Martia states smuggling is "an ancient and respected trade." Martia also says that the chameloids' talent at smuggling is due to frequent use of their shape-shifting abilities.
Chameloids were scripted to have been shown with green blood, in both Martia's most common form and her appearance as "the Brute". In the actual film, however, the blood is shown only in Martia's usual guise, disappearing when she morphs into Kirk. This raises the possibility that the blood was perhaps part of Martia's abilities to produce illusory appearances, possibly being illusory itself.
Martia's abilities were more extensive in the script with a change into a slavering tooth monster and a tentacle creature, but neither of these appeared in the final film.
In the final version of the film, there is still one defining characteristic of Martia's appearances, as actress Iman noted; "The only common denominator that we have is the eyes. Everything else I can change about myself but my eyes." (Original interviews: Iman, Star Trek VI: The Undiscovered Country (Blu-ray) special features) Special effects makeup artist Edward French decided that Martia's eyes should consistently remain the same, as a solution to director Nicholas Meyer being concerned that Martia be recognizable regardless of her appearance. "I designed the eyes on paper and Richard Snell made them from my sketches," French explained. (Cinefantastique, Vol. 2, No. 5, p. 34)
A non-canon source, Peter David's novel Imzadi, has a Chameloid character serving aboard a future version of the starship USS Enterprise. The character is described as tall and fur-covered, possibly resembling the "brute" form taken by Martia in The Undiscovered Country.