The Doctor's cat (Doctor Who?)

At some point in his travels, the Second Doctor got a cat that lived in him with the TARDIS. He wrote about feeding it in his Five Hundred Year Diary. (NOTVALID: Doctor Who? 114)

The Doctor still had the cat by the time of their sixth incarnation. It watched him eagerly explain the origins of his Dalek which was in use as a conversation piece hat rack to two of his party guests. (NOTVALID: Doctor Who? 124)

Behind the scenes
The Tenth Doctor would eventually gain a cat as a companion in Rose from the comic story A Rose by Any Other Name.