Dr. Jo'Bril, a Takaran.
The Takarans are a humanoid race native to the Alpha Quadrant planet Takara.
Takarans are characterized by a very unusual anatomy and physiology where their internal organs are not as distinct as those of most humanoids - their organ systems are much more evenly distributed around their bodies, resulting in Takarans having a very high resistance to injury. Takarans also have voluntary control over their own physiology down to a cellular level and can place themselves in a form of stasis resembling death. To Starfleet medical equipment, only the rate of cellular decay could verify whether a Takaran was dead or alive - a slow rate of cellular decay was the result of a Takaran putting themself into stasis; the rate of cellular decay would be much faster if the Takaran were truly dead - this was unknown to Starfleet medical officers until 2369.
Dr. Jo'Bril was a Takaran. (TNG: "Suspicions")
Takaran anatomy.
- Since Dr. Crusher knew little of Takaran physiology and also stated Jo'Bril was the first Takaran she'd ever met, it is unlikely that Takarans were Federation members as of 2369.