It’s forty years today since K-9 was left off the leash in The Invisible Enemy by Bob Baker and Dave Martin. The original robot dog, voiced by John Leeson, was the creation of Professor Marinus in the year 5000. Behind the scenes, it was a reaction to the popularity of R2-D2 and Star Wars in general that prompted his inclusion in Doctor Who.
Apart from Leeson, K-9 has also been voiced by David Brierley and Roy Skelton. There are several versions of the robot floating around the time vortex - Mk1 who remained with Leela on Gallifrey, Mk2 who stayed with Romana II in the N-Space intersection, and MkIII who was uncrated by Sarah Jane Smith in K-9 & Company. That last version was destroyed in an adventure with the Tenth Doctor who built MkIV. Another version appeared in an Australian television series. We may yet see another incarnation in a movie.