K9 (The Choice)

A version of K9 travelled with his Mistress in Ecto-Space. (AUDIO: The Choice, The Search)

In one version of the Mistress's timeline, (AUDIO: B.E.S. Begins) he "went off to do his own thing", leaving the Mistress travelling alone. She replaced him with the robotic ferret B.E.S. as her new companion. (PROSE: The Choice) In another version of history, she continued travelling with her canine companion and never met B.E.S.. (AUDIO: B.E.S. Begins, The Choice, The Search) The version of B.E.S. who travelled with the Mistress mentioned in her journal that the two pairs of time-crossed travellers once met due to a temporal anomaly, without elaborating. (AUDIO: B.E.S. Begins)

Behind the scenes
Bob Baker and Dave Martin gave BBV Productions permission to use this K9 for Adventures in a Pocket Universe. The story implies that this version of K9 is in fact K9 Mark II, who had remained in E-Space with Romana II after the events of Warriors' Gate, while the Mistress is implied to be Romana herself.

Because BBV no longer had the rights to use the character of K9, the 2022 novelisation of The Choice replaced his role in the plot with a different mechanical creature, the bionic ferret B.E.S.. The Mistress having had a previous, canine companion is alluded to in the novelisation and its prequel B.E.S. Begins, though he is not named.