This page gives some background information about Spot and is based on information found in the series "Star Trek The Next Generation" and the last released movie were Spot appeared in, "Star Trek: Generations".

Data's pet cat, Terran Felis domesticus, who lived with Data on the Enterprise-D. Spot was quite a gourmet, as Data created at least 221 different foods for her. A malfunction of the ship's replicator system resulting from an errant interface experiment in 2369 caused the ship's food slots to dispense cat food.
Commander Riker once agreed to care for Spot while Data was Away at a conference for three days. Unfortunately, Spot did not care for Commander Riker, and made her dislike evident. In 2370, Geordi "borrowed" Spot in an attempt to see if he wanted to have his own cat. Geordi was less than satisfied with Spot's visit to Geordi's quarters, Data undertook to train his cat, also with less than satisfying results.
While Spot's behavior continued unchanged, Geordi would note that data's "training" was "coming along nicely". The only person who Spot seemed to like was lieutenant Barclay, who thought Spot was "a sweet little kitty."
In 2370 Spot became pregnant. At the time, the father was undetermined, but Data intended to run a full DNA analysis of all 12 male cats on the ship to identify the sire. Spot also fell victim to the influence of Barclay's Protomorphosis Syndrome, apparently during the birth of her kittens. Spot devolved into a large iguana, but her kittens were unaffected. This led Data to devise a curse for the syndrome.
Spot survived the crash landing of the Enterprise-D saucer section in 2371. Upon discovering Spot was unharmed, Data, under the influence of Soong's emotion chip, shed his first tears of joy. Spot first appeared in the episode "Data's Day", but didn't get a name until the episode "In Theory". In"Data's Day", Spot was a Somali cat but in later appearances, Data's friend somehow became a common house cat. We speculate that Spot may be a shape-shifter or an unfortunate victim of a transporter malfunction. (From Star Trek Omnipedia)