Time zombies were creatures that once seemed to roam through the TARDIS.
They were in fact versions of the Eleventh Doctor, Clara Oswald, and Gregor & Tricky Van Baalen from the near future. They had been exposed to the TARDIS's Eye of Harmony to the extent that their cells had liquified, and their skin had burnt, Gregor and Tricky had also been fused together. Unlike appearances of "memories" from the recent past, the Doctor described these manifestations as "real."
When realising that their future could be rewritten, the Doctor escaped the Eye with Clara before they were transformed. He later sent back the message "Big Friendly Button" to a past version of himself, which prevented them all from being exposed, and thus no-one was ever transformed. (TV: Journey to the Centre of the TARDIS)
Notable members
Behind the scenes
Although Clara describes them as "zombie creatures", the term "Time Zombie" is actually taken from the episode's end credits.