Captain was a wild dog from the Middle Ages which the Fourth Doctor kept as a pet at Nest Cottage. He was later given to Mike Yates.
Biography
Meeting the Doctor
The Doctor met Captain in the Dark Ages in England. Captain led a pack of wild dogs possessed by the Hornets. Once freed from their control, Captain helped track them with the Doctor in the TARDIS. The Doctor took him to Nest Cottage as he had done with others the Hornets had possessed.
Leaving the Doctor
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.
Reunion with the Doctor
The following Christmas, Captain and Mike returned to Nest Cottage to visit the Doctor and Fenella Wibbsey. The two became embroiled in another of the Doctor's adventures. (AG: Hornets' Nest, Demon Quest)