In the current series, the Tenth Doctor gave Sarah Jane an upgraded Mark IV K-9 following the death of her original K-9, who followed her into her spin-off series The Sarah Jane Adventures.
The Doctor would rebuild K-9 three times over the course of his Fourth incarnation, sending a K-9 to accompany three of his female companions when they left his company – reporter Sarah Jane Smith, the warrior woman Leela and the Time Lady Romana.
A behind the scenes photo from the set of Doctor Who seems to show that the robot dog K-9 will be putting in an appearance in season 11.
Professor Marius gave K-9 to the Doctor in thanks for saving his life, and the robot dog quickly became the Doctor’s most loyal companion.