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).

Cultural significance
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 the incident aboard the USCSS Nostromo. However, it is possible that the Xenomorph did see the cat as a potential 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 it killed the Humans first it could eventually go and kill all the small harmless creatures secondarily, including the Cat.

In 1997, a Cat named 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 well-known astronomical location, Orion's Belt, in the eponymous constellation.

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 galaxy. A nice example would be the intelligent alien named Zunar-J-5/9 Doric-4-7, who landed on Earth in the year 1978 and communicated with Humans using a technological device on its collar.

Another example is that of the Thubanians, hailing from the dying planet Thuban IX, who arrived on Earth in ancient times and were considered sacred animals by the Egyptians.

The mysterious shape-shifting alien named Isis spends most of her time assuming the form of a black cat, keeping her real identity as a secret.

The Time Lords genetically engineered Cats so they could send then to other planets to be worshiped as gods (for colonization, I guess).

The mysterious Kitlings from the planet of the Cheetah People are also virtually identical to Earth's cats.

Humanoid aliens with feline traits are also fairly common in the universe, these including the venerable Kzinti, the Federation's Caitians, the shape-shifting Ctarl-Ctarl, the endangered Thunderians, the Khajiit from Elsweyr, the Cat-men from Callisto, and the Catkind from New Savannah. An unusual case of this is that of the Felis sapiens; a Cat-derived race which literally evolved into a humanoid life form over three million years.

Although not actual cats, there are cat-like beings from the Saturnian Dreamlands known simply as the Cats from Saturn; which are known for their ongoing blood feud against Earth cats. They can sometimes be found visiting the Lunarian Dreamlands where they have allied themselves with the native Moon-Beasts.