Vendorian

From Memory Alpha, the free Star Trek reference

The Vendorians are a cephalopodic shapeshifting non-humanoid species that have perfected the techniques of cellular metamorphosis, and are capable of rearranging their own cellular structures to imitate any person or object of similar size and mass. While Vendorians may assume many shapes and identities, the longer they remain in one form the more characteristics of that form they assume. This is particularly true when a Vendorian assumes a humanoid form. In these instances, the Vendorian absorbs memories and personality traits of an individual through touch. These memories and traits become more ingrained the longer or more frequently the Vendorian assumes the form; if maintained long enough, the absorbed traits eventually become part of the Vendorian's own personality.

As they demonstrated in their ultimately unsuccessful efforts to save the life of trader and philanthropist Carter Winston, Vendorians are excellent surgeons – due in no small part to having six tentacles in their true forms – and likewise possess knowledge of other branches of medicine.

Vendorians also have the ability to render some humanoids, particularly Humans, unconscious by touch alone.

It is unknown whether this process is chemical/electrical or mechanical, such as the Vulcan nerve pinch. Though it was demonstrated several times on-screen, no explanation was ever provided.

Unfortunately for communities of worlds, the Vendorians practice deception as a way of life and have been deemed psychologically unfit to participate in such communities on those grounds. Indeed, due to the Vendorians' deceitful way of life, the Federation imposed a quarantine on their homeworld, Vendor, declaring it off-limits to travelers.

Under the terms of the treaty between the Federation and Romulan Empire, use of Vendorian spies was considered a violation of the treaty and hence an act of war. (TAS: "The Survivor")

It is unknown if this refers to the treaty established following the Earth-Romulan War of 2161, or the more recent Neutral Zone Treaty.