Rare shapeshifters able to assume almost any humanoid form, so seldom seen that James T. Kirk believed them to be a myth.
Chameloids can change their appearance at will, imitating people of different species and sexes. Before meeting one, Kirk assumed they existed only in legend.
In Star Trek VI: The Undiscovered Country, Kirk and McCoy were sentenced to the Klingon prison Rura Penthe, where the chameloid Martia helped them escape. She was secretly working for the commandant, who meant to have them killed while fleeing, and she died after taking Kirk's form. In Star Trek: Section 31, the operative Quasi is also a chameloid.