Spot is Data's cat. Spot is a way we the viewers can watch Data grow as a person while raising another lifeform. We also got to watch entire minutes of the show devoted to Data's poetry about Spot, which was some tedious shit.
It is interesting to note that Arik Soong, the mad genetic doctor and part-time actor who played Data, was as fond of cats as he was of kicks to the groin. If one watches TNG when it features Data with Spot one can often see the pure disdain on Data's face for the feline (Genesis, for example).
Spot became pregnant onboard the Enterprise. Data noted he did not know who the father was as Spot had escaped numerous times and there were twelve male cats on the ship. I believe this further proves that security on the Enterprise was about on par with my father's seldom-locked garage. Numerous lifeforms roaming unchecked on the flagship of the fleet - I can think of several ways to exploit that using biologic agents alone. No wonder Picard didn't ask for Worf back.