I nonetheless consider you a true and valued friend."
Spot was a female cat and Data's pet in the 2360s and 2370s. She lived with him while he served aboard the USS Enterprise-D and -E. She had a litter of kittens with one of the other cats on the Enterprise-D. Data once wrote a poem about Spot, entitled "Ode to Spot". (TNG: "Schisms", "Genesis")
As a starship cat, Spot had never encountered rodents, insects or been chased by a canine. (TNG: "Phantasms") Nevertheless, Spot was an active cat and received a lot of attention from Data, often playing with him and jumping on his desk for attention. (TNG: "Data's Day", "A Fistful of Datas", "Force of Nature", "Descent, Part II") Spot also left Data's quarters to explore, though she could not trigger the doors. (TNG: "In Theory") Spot had tried 221 different formulas of food designed by Data, many of which she did not care for, but she did like his supplement 25. (TNG: "Phantasms", "In Theory", "A Fistful of Datas") Spot also was observed sleeping by Data, during which time she underwent muscle spasms. (TNG: "Phantasms") Spot also appeared in Data's dreams. (TNG: "Birthright, Part I")
Data unsuccessfully tried to train Spot, since she often tried to get attention when he was working. He used several methods including conditioned-response exercises, environmental enhancement, and bioconditioning devices, such as sensor nets for behavioral modification or biofeedback motivators. Upon noticing how readily Data would respond to Spot's requests for attention and playtime, Geordi La Forge joked that it was actually Spot who was successfully "training" Data. On another occasion, when Worf tried to give Spot commands, Spot did not respond to them. (TNG: "Timescape", "Force of Nature", "Genesis")
Spot was a relatively fickle cat with people, and only seemed to get along well with Data, Dr. Crusher, Deanna Troi, and Reginald Barclay, which the latter found surprising. In 2369, while taking care of Spot, Commander Riker was forced to go to sickbay when she scratched his face. The next year, La Forge borrowed Spot, since he wanted to see what having a pet cat was like. They did not get along well, either. In 2370, Worf briefly took care of her, though he found himself allergic. He simply fed her. (TNG: "Timescape", "Force of Nature", "Genesis", "Phantasms")
In 2370, Spot became pregnant by one of the twelve male cats aboard the Enterprise-D at that time. Data paid attention to her health, including bringing her to sickbay for a check-up just before giving birth. (TNG: "Genesis")
Later on, Spot played an integral part in saving the Enterprise crew. While Data and Jean-Luc Picard were on an away mission, a synthetic T-cell became airborne and infected the crew and the animals. This caused inactive parts of their genetic code to activate and essentially "de-evolve" them, including Spot. She gave birth to her litter while also de-evolving into a reptile. When Data and Picard returned, they found her and saw that her kittens were unaffected. They realized that the natural defense mechanisms in the placenta, which protect infants from the mother's diseases while in the womb, might prove useful in creating an antidote. This led Data to develop a cure using amniotic fluid from Nurse Ogawa, who was also pregnant at the time. (TNG: "Genesis")
In 2371, Spot survived the destruction of the Enterprise-D. Data, via his newly installed emotion chip, cried tears of joy over Spot's survival. Spot continued to live with Data until his death in 2379. Spot then met B-4 aboard the Enterprise-E. (Star Trek Generations; VOY: "Pathfinder"; Star Trek Nemesis)
Spot first appeared in "Data's Day" as a male, long-haired Somali cat. (citation needed • edit) According to Richard Arnold, Spot was initially a male Abyssinian cat. The reason the cat's gender was inexplicably changed was, in reality, that Jeri Taylor decided Spot should have kittens. Also, the cat which first portrayed Spot died and was then replaced by a pair of other cats. (Star Trek: Communicator issue 101, p. 15) In subsequent appearances, Spot was seen as a more common American short-hair orange tabby, but still as a male. It was only in the seventh-season episode "Force of Nature" that Spot was first referred to as "she". In "Genesis", she even gave birth. The Star Trek Encyclopedia (3rd ed., p. 460) jokingly suggests that Spot may be a shapeshifter or the victim of a transporter malfunction. Brent Spiner joked that there were actually 3,500 cats on the Enterprise-D and Data referred to all of them as "Spot". David Arnold suggested that Data perhaps got another cat and simply kept the name. (Star Trek: Communicator issue 101, p. 15)
Spot was played by unknown long-haired Somali cats in her first two appearances and trained by Gary Gero and Scott Hart from Birds & Animals Unlimited. Spot's appearance was changed into an orange tabby cat beginning with the sixth season when Rob Bloch from Critters of the Cinema took over the animal casting and training. During her time on The Next Generation, Spot was then played by Monster, Brandy and Bud and later also by Tyler, trained by Rob Bloch and Karen Thomas-Kolakowski. (citation needed • edit) Monster and Brandy were a pair of twin tabby cats and it was they who replaced the first cat to have played the role of Spot. (Star Trek: Communicator issue 101, p. 15) In "Genesis", when she turned into a lizard, the iguana portraying her was named Willie. 
In the first draft script of Star Trek Generations, Spot ended up on the holodeck while the holoprogram of the brig USS Enterprise was running, prompting Riker to call to Data, "Will you get your damn cat off the Holodeck!"
When it was decided that Spot would appear in Star Trek Generations, Critters of the Cinema trainers Bloch, Thomas-Kolakowski and Kelli Cole trained the cats Monster and Brandy and their back-ups Spencer and Zoe. While Monster appeared in the scenes earlier in the film, when a more spunky Spot was required, Brandy was the cat seen at the end of the movie. Critters of the Cinema was not called back for the production of Star Trek Nemesis. (citation needed • edit)
A scene that was cut from Star Trek Nemesis revealed that Worf was left to care for Spot. In the scene, Worf and La Forge were cleaning out Data's quarters. Worf heard a noise, and looked up. Immediately after, Spot jumped into his arms. Worf stated that he "was not a cat person." La Forge then said, "You are now." This scene, along with other deleted scenes, was added as an extra on the film's Special Edition DVD.
While the character of Data loved Spot, Brent Spiner may have developed a minor dislike for the cat. In the commentary of Star Trek Generations, it was stated that he did not want to go back for Spot, and during an interview in the 2010 Wizard World in Philadelphia, he said, in a joking manner, that he thought the cat was "the stupidest actor I've ever worked with," saying that he "never took a piece of direction, ever," except for one time where "they" (presumably the writers) wrote in, "Spot eats tuna out of a can." Spiner added, "And he did it on the first take. Otherwise, we were there all day with that cat."
In the TNG short story "On The Spot" from the anthology The Sky's the Limit, following the events depicted in the deleted scene where Spot jumps into his arms, Worf assumes the role of caretaker for Spot after Data's death. Worf, after Spot assists with hunting down an alien infestation on the Enterprise, states that he finds her to be a warrior, who simply lulls people into a false confidence as a cat.