Harvongian
A Harvongian was a type of shape-changer known to the Federation by the latter half of the 24th century.

History 
Sometime during the 2370s, the  was infiltrated by a Harvongian, who appeared in the guise of a Human male named Niko. That year, Niko and his girlfriend Angie, along Beckett Mariner, were off-duty together when, while discussing how great Niko was, Mariner joked to him that he was either "the nicest, smartest, handsomest guy on this ship [..] or you're a Harvongian shape-changer." As soon as she mentioned the name, it appeared to trigger a reaction from the creature, as Niko became entranced, and his eyes popped out as the skin of his body split bilaterally as the snarling Harvongian within exposed its true identity and immediately grabbed Angie in its mouth and began to consume her. Many members of the Quitos crew began firing phasers at the creature, presumably neutralizing it. 

Nearly a decade later, when Mariner discovered that Brad Boimler was in a relationship with Barbara Brinson, she grew concerned, citing that "when a Starfleet relationship seems too good to be true, then [...] it probably is," before drawing several possible conclusions of Brinson's origins, including her possibly being "a secret alien who's gonna eat you", before stating, "I have seen stuff, man," before recalling her experience on the Quito, finishing, "I'm not gonna sit around and let a face get melted. Not again." ()

Biology 
In its true form, a Harvongian shape-changer appeared reptilian, with green skin, large forearms with three clawed fingers, an additional smaller set of arms protruding from mid-torso tipped with a single talon. They had a proportionally oversized snout and maw, which contained large fanged teeth, and three prehensile tongues, that were over two meters long, capable of manipulating and lifting an average-sized humanoid body for consumption. They were also tolerant to damage caused by multiple type 2 phaser hits. ()

Species