Professor Frederick Marius, who invented the first K9 in the year 5000 while working on the asteroid K4067, described him as "my best friend and constant companion." Marius had a dog on Earth, but weight requirements did not allow him to bring his real dog into space, so he built K9.
(TV: The Invisible Enemy) He built K9 by using the latest technology.
(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) This K9 exhibited the ability to sense and warn others of danger.
Immediately after leaving Mark I behind with Leela, he unpacked K9 Mark II.
This version of K9 accompanied the Doctor and his new companion on their quest to locate the segments of the Key to Time, (TV: The Ribos Operation) but the swampy terrain of Delta III made it impossible for him to join them on the planet.
(PROSE: Tautology) The Professor offered K9 to the Fourth Doctor as the same weight requirements made him unable to take Mark I back to Earth.
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: The Ribos Operation) It is possible that the Doctor spent some time at Nest Cottage before activating him.
(AUDIO: The Stuff of Nightmares) There are accounts that this model of K9 went on missions on behalf of the Time Lords and was given control of a space vessel called K-NEL.