Spot

Spot belonged to Data, the android officer aboard the USS Enterprise-D, and served a rather more sophisticated purpose in Star Trek: The Next Generation than most television pets. Data spent seven seasons trying to understand human emotion from the outside, and the writers gave him a cat — an animal that is famously indifferent to whether you understand it — as the vehicle for a good deal of that exploration.

The cat's continuity was famously loose. Spot first appeared as a male Somali, was referred to as male in early episodes, and later became a female orange tabby who gave birth to kittens, with no in-universe explanation ever offered. Several different cats played the part over the years. Fans have been cheerfully complaining about this for three decades, which is itself part of Spot's appeal.

The character produced one of the series' best-loved comic moments in the fifth-season episode "Schisms", where Data reads aloud his composition "Ode to Spot" — a poem of genuine affection rendered entirely in technical vocabulary, praising his cat's "cranial capacity" and "sublime tetrapedal grace" in a flat android monotone while the assembled crew struggle to keep straight faces. It works because the joke is affectionate rather than cruel: Data plainly loves the cat, and is simply reaching for the only register he has.

Spot's most significant appearance came in the film Star Trek: Generations, where the Enterprise-D is destroyed and Data, who has recently installed an emotion chip, discovers Spot alive in the wreckage and bursts into tears — the first genuine, uncomplicated feeling he has ever experienced. It is the payoff to seven years of setup, and the writers chose a cat to deliver it.

For a franchise built on questions about what makes a person, a cat turns out to be a well-chosen prop. You cannot reason a cat into affection, and a being trying to learn how to feel could hardly ask for a more demanding teacher.