Cat

"Felis catus is your taxonomic nomenclature An endothermic quadruped, carnivorous by nature."

- Data in his poem, Ode to Spot

Cats (Felis catus) are small predatory animals that can be found at least on two different planets: Earth (the third world of the Sol System, in the Milky Way Galaxy) and Melmac (assumed to be located in the Andromeda Galaxy).

Cats are agile and clever hunters and their menu consists of all sorts of small creatures, including pests like rats, which is why they are widely kept as pets by Humans. On Melmac, on the other hand, Cats are considered a delicacy and they are a favorite dish of many Melmacians. It is currently unknown if the Earth Cats and the Melmac Cats belong to the same species.

It appears that Cats remain relatively unharmed when near Xenomorphs as seen in Alien. However, it is possible that the Xenomorph saw the cat as a food source, but opted to pursue the humans first either due to them being a greater source of food or posing a greater threat, figuring that if they killed the humans first they could eventually go and kill all the small harmless creatures secondarily like the cats.

A cat called Orion, which had belonged to a member of the Arquillian royal family, was pivotal in preventing Earth's destruction by the Arquillians, as it carried a miniature galaxy on its collar which was desired by both the Arquillians and the Bugs. The information that the galaxy was located in "Orion's belt" tricked some people into believing that this was referring to the astronomical location known as Orion's Belt.

In the 24th century, a cat called Spot is a pet owned by the android Data, lieutenant commander of the starship USS Enterprise-D. Despite having no emotions, Data is quite attached to his cat and has even written a poem for her, "Ode to Spot".

Similar Alien Species
Sapient extraterrestrial species which are physically indistinguishable from normal cats, as of outside appearance, are not unheard of in the science fiction field. A nice example would be the 1978 movie The Cat from Outer Space, in which an intelligent alien cat arrives on Earth and communicates with Humans using a technological device on its collar.

The Thubanians from the Futurama episode "That Darn Katz" are another example. Hailing from the dying planet Thuban IX, they arrived on Earth in ancient times and were considered sacred by the Egyptians.

A mysterious shape-shifting alien called Isis, from the Star Trek episode "Assignment: Earth", spends almost all the time in the form of a black cat, keeping her real identity as a secret.

Humanoid aliens with feline traits are also fairly common in sci-fi, these including Larry Niven's Kzinti, Star Trek 's Caitians, The Elder Scroll series' race called Khajiit, and Doctor Who  's Catkind. An unusual case of this was in Red Dwarf, in which the protagonist's cat literally evolves into a humanoid life form over three million years.

Although not actual cats, there are cat-like beings from Cthulhu Mythos called the Cat from Saturn and Cat from Uranus.