Captain was a wild dog from the Middle Ages whom the Fourth Doctor kept as a pet at Nest Cottage. He was later given to Mike Yates.
The Doctor met Captain in England in the Dark Ages. Captain led a pack of wild dogs possessed by the Hornets. Once freed from their control, he helped the Doctor track them in the TARDIS. The Doctor took him to Nest Cottage as he had done with others the Hornets had possessed. (AUDIO: A Sting in the Tale)
The Doctor gave Captain to Mike Yates to look after once the Hornets had been defeated. The Doctor already had a dog aboard the TARDIS and he did not think the two would get along. (AUDIO: Hive of Horror)