Antranak (cat)

Antranak was a stray cat Erimem brought aboard the Fifth Doctor's TARDIS.

Biography
Antranak was one of many stray cats who lived in abandoned tunnels near Thebes, Egypt. Erimem adopted the cat, who had in the course of events absorbed a malevolent alien intelligence harmlessly into its mind. She brought him with her when she left Egypt to travel with the Doctor. (BFA: The Eye of the Scorpion) Erimem named the cat Antranak, after her mentor. (BFA: The Church and the Crown)

Erimem and Peri were quite fond of Antranak, the Doctor less so. Antranak quickly made himself at home in the TARDIS, much to the Doctor's chagrin, thanks to the cat's habit of jumping onto the console and refusal to use the litterbox. (BFA: No Place Like Home)

Departure
Antranak followed the TARDIS crew onto the planet Talderun, where an energy converter had been built. The converter supported the consciousness of its creator, Shara, in a heavenly state of non-existence in a pocket universe while his body anchored the reaction on the material plane. Due to the theft and destruction of Shara's corpse, the reaction was becoming critically imbalanced, and would soon destroy the planet and then the entire star system.

It could only be rebalanced by another living being mounting an altar and swapping places with Shara. Shara would return to the physical world and die while the sacrificed being would be trapped in the pocket Universe. The Doctor prepared to sacrifice himself to save the planet, but Erimem insisted she should be the one to make the self-sacrifice. While they argued, Antranak leaped onto the altar, swapping places with Shara, stabilising the reaction, and saving the planet. Antranak's consciousness went into the pocket universe while Shara, in Antranak's body, died in peace.

The Doctor later wondered if the alien creature in the cat's mind forced him to act. Erimem was firm in her belief that her noble cat sacrificed himself. (BFA: Nekromanteia)