The Eighth Doctor Adventures novel Interference: Book Two by Lawrence Miles indicates that the Doctor built a Mark IV model sometime prior to the events of that book, but what happened to this unit is not stated. In the novel The Gallifrey Chronicles by Lance Parkin, K9 Mark II makes another appearance, having been trapped within the TARDIS since the events of The Ancestor Cell. At the end of The Gallifrey Chronicles the Doctor sends K9 on a secret mission to Espero, presumably to seek out his former companion, the living TARDIS known as Compassion.
After K9 Mark I regenerated into K9 Mark 2, his design was radically altered. His ear probes were replaced with larger silver aerials. A set of blue lights on his head flashed when he spoke. Most of his body was dark silver and a silver dog bone was located under his neck. He seemed to have the same capabilities as Mark I but was presumably more advanced. (TV: Regeneration)
The Fourth Doctor found that K9 had contracted laryngitis and had lost his voice as a result of this. K9 was left in the TARDIS while the Doctor and Romana II battled the Daleks and Davros. (TV: Destiny of the Daleks) During a later adventure, he remained patiently in the TARDIS, which would imply that his voice had not yet returned. (TV: City of Death) After his voice returned, it sounded quite different and remained so for a while (TV: The Creature from the Pit) before the familiar voice returned. (TV: The Leisure Hive)
K9 was not originally intended to be a companion, but producer Graham Williams liked the concept so much that the decision was made to retain him as a regular character, in order to appeal to the younger members of the audience. The original name for the character was "FIDO" — apparently from "Phenomenal [sic] Indication Data Observation" unit — but it was eventually named K9.
Within the series' narrative, K9 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, K9 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 K9 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 K9's popularity with children would launch the series K9 and Company, led by actress Elisabeth Sladen in the role of companion Sarah Jane Smith alongside a new K9. In 2006, K9 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 K9 spin-off, the character began to appear full-time in both K9 and The Sarah Jane Adventures, depicting adventures of the original K9 Mark I and of Mark IV, respectively.