Star Trek has featured many different kinds of animals who have been pets aboard various starships. Nearly every Star Trek series has been set aboard a Starfleet ship, or a space station in the case of Star Trek: Deep Space Nine, but traveling millions of lightyears from Earth (or their various homeworlds) hasn't stopped several Star Trek characters from keeping beloved pets as companions on their extended voyages throughout the final frontier.
Star Trek: The Next Generation popularized the idea of pets on a starship; the Galaxy-class U.S.S. Enterprise-D had a crew complement of 1,000 Starfleet Officers but this also included their families, who lived aboard the enormous vessel as well. The Enterprise-D was essentially a small town in outer space; it had a school for the children and they also celebrated Captain Picard Day once a year. Naturally, there were also pets aboard the Enterprise-D (as well as dolphins in Star Trek's Cetacean Ops to help navigate the starship). Some of the Enterprise-D's senior officers like Lt. Commander Data (Brent Spiner) and Captain Jean-Luc Picard (Patrick Stewart) himself had pets aboard. The other Star Trek series and movies followed suit and soon introduced other pets aboard the different starships.
There are also other animals throughout Star Trek who have been pets but didn't live aboard starships with their owners. These include Mollie, the Irish Setter owned by Star Trek: Voyager's Captain Kathryn Janeway (Kate Mulgrew), who she left behind on Earth with her then-fiance, the sehlat (a fat teddy bear with 6-inch fangs) Spock (Leonard Nimoy) owned as a child on Vulcan, Neelix, the cat owned by Lt. Reginald Barclay (Dwight Schultz) when he worked at the Pathfinder Project on Earth, Chief Miles O'Brien (Colm Meaney) and his family's cat named Chester on Deep Space Nine, and Number One, the pitbull Admiral Jean-Luc Picard owned when he retired to his family vineyard in La Barre, France, in Star Trek: Picard.
But the real action for Star Trek's pets was in outer space and here are the different animals (and other creatures) who got to live the adventure in the final frontier aboard a starship.
Porthos - Star Trek: Enterprise
Porthos was Captain Jonathan Archer's (Scott Bakula) adorable beagle and he kept his owner company aboard the NX-01 Enterprise throughout the entirety of Archer's historic decade-long mission. Porthos literally got to go "where no dog has gone before" in the 22nd century, although the poor dog often got sick from the Captain feeding him cheese when he shouldn't. Porthos also had his pituitary glad replaced with a Calrissian chameleon's after he contracted an alien pathogen.
In the alternate Kelvin timeline, Montgomery Scott (Simon Pegg) mentioned that he tested transwarp beaming on "Admiral Archer's prized beagle". Since J.J. Abrams' Star Trek 2009 took place about a century after Star Trek: Enterprise, this meant that Scotty risked the life of a different beagle since Porthos would have been long dead.
Dr. Phlox's Pyrithian Bat - Star Trek: Enterprise
Dr. Phlox (John Billingsley) was the Chief Medical Officer aboard the NX-01 Enterprise and he kept a menagerie of strange alien creatures aboard for medicinal purposes. But Phlox's favorite pet was his female Pyrithian bat, which he spoke to often. The bat made several appearances in Star Trek: Enterprise, and it even got loose when Captain Archer had to spend a night in Sickbay with Porthos, but Dr. Phlox never did give his Pyrithian bat a name.
Tribbles
Tribbles have made several appearances in Star Trek and many Starfleet Officers have made the misguided mistake to try to keep the furry creatures as pets, which usually ended up with the Tribbles breeding and infesting their starships. In the Short Trek "The Trouble With Edward", which was set years before Star Trek: The Original Series, the U.S.S. Cabot under the command of Captain Lynne Lucero (Rosa Salazar) was overrun with Tribbles after they were genetically modified by Edward Larkin (H. Jon Benjamin). In Star Trek: Discovery season 1, Captain Gabriel Lorca (Jason Isaacs) kept a Tribble in his ready room.
The Tribbles' first real-world appearance was in the TOS classic "The Trouble With Tribbles" when Lt. Uhura (Nichelle Nichols) tried to keep a Tribble as a pet aboard the Enterprise and they multiplied in the ship and on the K-7 space station, which was also seen from a different perspective in the DS9 episode "Trials and Tribble-ations".
Ripper the Tardigrade - Star Trek: Discovery Season 1
A gigantic tardigrade was briefly kept aboard the U.S.S. Discovery in Star Trek: Discovery season 1. At first, the tardigrade, named Ripper, was treated like a monster and it killed Discovery's Security Chief, Commander Ellen Landry (Rekha Sharma). However, Michael Burnham (Sonequa Martin-Green) realized the tardigrade could become the interface for the Discovery's spore displacement hub drive. Ripper briefly became the Discovery's navigator along the mycelial highway, but it was slowly dying from the burden. Burnham set Ripper free and the tardigrade vanished into the mycelial network.
Kruge's Klingon Dog - Star Trek III: The Search For Spock
The first pet to appear on a starship in a Star Trek movie was, surprisingly, the lizard-like Klingon dog owned by Commander Kruge (Christopher Lloyd) in Star Trek III: The Search for Spock. Identified as a "monster dog" in the screenplay, Kruge's intimidating pet with razor-sharp teeth lived aboard his Bird-of-Prey and was chained on the bridge next to his Captain's chair. Kruge's monster dog died when the Enterprise hit the Bird-of-Prey with photon torpedoes, causing a power overload that electrocuted the beast.
Spot - Star Trek: The Next Generation
Spot was Lt. Commander Data's cat and caring for him was a crucial component of the android's quest to become more human. Spot first appeared in the TNG season 4 episode "Data's Day" and he was part of several more episodes, as well as two of the TNG movies. Spot survived the crash of the U.S.S. Enterprise-D in Star Trek Generations, which brought Data, who installed his emotion chip, to tears of happiness. Spot also lived with Data aboard the Enterprise-E in Star Trek: Nemesis.
Captain Picard's Fish And Miles O'Brien's Tarantula - Star Trek: The Next Generation
Captain Picard kept a spherical aquarium installed in the wall of his ready room on the Enterprise-D but the most notable of his pets was a lionfish named Livingston. Another pet kept by one of the Enterprise's officers was Christina, a Lycosa tarantula adopted by Chief O'Brien on Titus IV. The tarantula crawled up Reg Barclay's arm in Ten Forward but it wasn't seen again after the TNG season 6 episode "Realm of Fear".
The Dog - Star Trek: Lower Decks
Despite its name and outward appearance, The Dog is not a dog. Rather, it's a genetically-engineered lifeform created by Ensign D'Vana Tendi (Noel Wells) on Star Trek: Lower Decks. Tendi was incredibly proud of creating The Dog from inert carbon and she hand-edited all 6-billion sequences encoded in her DNA. Unfortunately, as an Orion, Tendi had no idea what a dog from Earth actually was so she built The Dog to be a shapeshifter that could fly, speak, and emit bats from its teeth-filled maw. The Dog was taken by Division 14 to the medical spa called The Farm, where it could be cared for by Starfleet specialists.
Grudge - Star Trek: Discovery Season 3
The newest pet in Star Trek canon is Grudge, the cat owned by Cleveland "Book" Booker in the 32nd century. Described as "a very large cat" by Michael Burnham, Book explained that Grudge has a thyroid condition (although the cat is played by Maine Coon, which explains her size). Regardless, Grudge is Book's companion aboard his ship during his galactic travels as a courier in Star Trek: Discovery season 3, and Book is completely devoted to her, referring to Grudge as "a queen".