K9

K-9 was the first - and until Kamelion, the only - robotic companion.

In fact, there were three K9s. K9 Mark I was built by Professor Marius to accompany and assist him in his work at the Bi-Al Foundation. This K9 joined the Doctor and Leela in The Invisible Enemy, but left with Leela at the end of The Invasion of Time.

The Doctor then built a new K9 (Mark II), more or less identical to the original, which accompanied him and Romana from The Ribos Operation to Warrior's Gate, when it left with Romana.

The Doctor also at some point built K9 Mark III, which he left on Earth as a present for his former companion Sarah Jane Smith. This K9 appeared in the spin-off adventure K9 and Company, and also in the 20th anniversary special The Five Doctors.