For those who may not have watched Doctor Who, or have only seen recent episodes, here is a bit of explanation from Wikipedia about K-9.
K-9 is a robot dog acquired by Doctor Who's title character in the 1977 serial The Invisible Enemy. The first two incarnations of the character travelled alongside the fourth Doctor (portrayed by Tom Baker) until 1981. In these stories, K-9 proved useful for the powerful laser weapon concealed in his nose, his encyclopaedic knowledge and his vast computer intelligence. By 1981, each of the two models of K-9 which travelled alongside the Doctor had been left with one of the Doctor's female companions. The character subsequently transitioned into spin-off territory. Producers hoped K-9's popularity with children would launch the series K-9 and Company, led by actress Elisabeth Sladen in the role of companion Sarah Jane Smith alongside a new K-9. In 2006, K-9 reappeared in the revived series of Doctor Who. Although the character's appearances in the 2007 spin-off The Sarah Jane Adventures were hampered by the concurrent development of the K-9 spin-off, the character began to appear full-time in both K-9 and The Sarah Jane Adventures, depicting adventures of the original K-9 Mark I and of Mark IV, respectively.
When I began watching Doctor Who as a kid on PBS here in the states, Tom Baker was starring as The Doctor. I watched Doctor Who religiously when it was on the air and have done so through to the new season that is currently starring Peter Capaldi. I adored the episode during Tennant's tenure when Sarah Jane Smith showed up with K-9.
This video showcases the four models, not including the Australian spinoff series K-9 had.