Ornithoid Lifeform

"Ornithoid Lifeforms" is a designation used to refer to a mysterious sapient species of diminutive beings native to a galaxy other than the Milky Way. The two individuals discovered on the planet Pyris VII were exploring the Milky Way, under the orders of a superior power which they refer to as the "Old Ones", whose exact nature is never made clear.

Biology
These aliens are enormously advanced and experience physical sensations differently from Humans, due to radical differences of the environmental conditions under which each species evolved. They also aren't physically suitable for surviving on Earth-like environment in their true forms.

In their natural form they resemble a strange kind of bipedal insect with fluff-like blue fur covering their bodies. Their heads sport tentacles and antennae. They are rather small creatures (less than a foot tall) and their fragile bodies easily die under Human-suitable environments. When they perish, their bodies quickly break down, with smoke coming out of them, suggesting that their tissues are vaporizing.

Technology
Their technology is based around the transmuter: a crystalline device which allows them to highly amplify their natural mental abilities, giving them the powers to travel between galaxies, to read and control minds, to exert control over matter over vast distances, and to alter their bodies in order to achieve a form most appropriate for the environment they find themselves in. In short: the transmuter allows them practically unlimited control over mind and matter.

History
Two members of this species, Korob and Sylvia, traveled to the Milky Way in a mission, apparently sent by the so-called Old Ones, and established themselves on the planet Pyris VII, where they made contact with three crewmembers of the USS Enterprise: Scotty, Sulu and Jackson; who had been sent on an away mission on the planet. The Ornithoid Aliens probed the minds of the men and created a scenario using of the subconscious images they found, mistaking Human folklore and cultural impressions for a realistic sample of their environment. The result was a dark, eerie fog-covered area surrounding a haunted castle with spectral witches, black cats and skeletons, and other strange phenomena.

This first contact ended in disaster, with Scotty and Sulu being held captive and having their minds controlled by the aliens, while Jackson escaped and beaming up to the Enterprise, already insane, only to collapse and die upon reaching the ship. Following this, Captain Kirk, Spock and Dr. McCoy decide to beam down to the planet to investigate, and meet with the two aliens. Soon, McCoy is also hypnotized.

As Kirk finds out, Korob and Sylvia have severe disagreements over how to proceed: the former claims loyalty to the Old Ones and wishes to move on with their mission (the exact nature and purpose of which is never made clear), while the latter takes pleasure in the new sensations provided by the humanoid form and wants to remain there. She later proposes to Kirk that he joins her and that they get rid of Korob and steal his transmuter. Kirk refuses, and Sylvia gets dangerously mad, creating a giant black cat to attack Kirk and the others. Korob is killed, but Kirk manages to salvage his transmuter and shatters it, nullifying all previous effects. The planet is back to normal, so are McCoy, Scotty and Sulu. Korob and Sylvia revert to their original Ornithoid bodies and dissolve.

Appearances

 * Star Trek - S02E07 - "Catspaw"

Behind the scenes
No proper name was given to refer to this species in their episode. The name "Ornithoid Aliens" and the diagrams detailing their anatomy above originate from the Starfleet Medical Reference Manual, in which these creatures are classified as vita xenos: a denomination used to refer to extra-galactic species (also including for instance the Kelvans and the Blastoneurons). The Manual also claims that these aliens hail from a cold, dry planet, with gravitational pull equaling one sixth of Earth's and a low atmospheric pressure. It further elaborates that the straw-like mouth and simple digestive system indicates that the species is probably liquivorous.

The anatomy diagrams (labelled as "Ornithoid Lifeforms") are later featured on the DS9 school's computer screen and students' PADDs in several episodes of Star Trek: Deep Space Nine; arguably making them canon.