Ecidien Cerebus Bird was an inhabitant of Ecidius IV. At some point, it was transported to the Doctor's TARDIS. It was able to hold a memories of a being it was telepathically linked to.
When Romana I was in the depths of the TARDIS she became confused at hearing its voice and K9 Mark II couldn't detect it due to the dimensional forces in the TARDIS. She confronted the Fourth Doctor about it but the Doctor but he dismissed it but he started to experience time spillage. (AUDIO: The Sands of Life) Romana II and K9 later heard it in one of the corridors of the TARDIS again confronted the Doctor but he dismissed it once more, she was determined to find it source. (AUDIO: The Trouble with Drax) She eventually found it in the depths of the TARDIS in the room which mimicked the Bird's natural habitat. She started to realise it was connected to the Doctor and tried to get the Doctor to admit it was there. (AUDIO: The Pursuit of History) When the Doctor's mind was wiped by the Conglom-Net computer whilst trying to save the Laan, it came from the TARDIS and restored the Doctor's memory. The Doctor finally admitted to Romana that he knew it was there all along but the telepathic connection he had with it made him forget it was there. He wasn't sure how long it had been there and who had put it there, but mused whether it was the White Guardian. (AUDIO: Casualties of Time)
Upon exploring the TARDIS Charlotte Pollard saw it and mentioned to the Eighth Doctor. Charley inquired about it and the Doctor told her that if she played her cards wrong he would tell her about it. (AUDIO: The Light at the End)