Professor Frederick Marius (PROSE: One Man and His Dog) was a scientist who worked at the Bi-Al Foundation in the year 5000 as a specialist in extraterrestrial pathological endomorphisms. He was the creator and first master of K9 Mark I.
According to one source, Marius was given a gift of a broken-down computer from the 20th century by his niece's fiance. Marius was extremely surprised by how technologically advanced the machine was, especially since it had come from the 20st century. The machine was actually an older model of K9 once owned by Sarah Jane Smith. Marius was forced to destroy the machine due to a lack of time, but build the circuitry of K9 Mark I based heavily on the machine. (PROSE: Tautology)
Marius had a dog, although sources dispute the name of the animal. By some accounts his name was Toby, (PROSE: Tautology) and by others he was known as Kelso. (PROSE: One Man and His Dog) He could not to take the dog to his new workplace at the laboratory of the Bi-Al Foundation. (PROSE: Tautology, PROSE: One Man and His Dog) According to one source, Marius left Kelso with Grace and constructed K9 to accompany and assist him in his work. (PROSE: One Man and His Dog) According to the other source, Marius would keep Toby's nametag and would place it around K9's neck. (PROSE: Tautology)
He helped the Fourth Doctor defeat the Swarm, although he himself was briefly infected by it and so freed the Nucleus and made it grow in size. Afterwards, Marius gave K9 to the Doctor to take with him on his journeys. (TV: The Invisible Enemy)