Pod person

Bernice Summerfield classed xenomelian species like the Silvans as "pod people", as they were parasitic replicators. When they invaded Kellor, 60 years before Benny arrived, the Silvans disguised themselves as the native Kellorans, by duplicating them, and replaced each individual with one of their own.

When finally confronted by the Silvans' infrantry, the Unbound Doctor imagined that "It'll be Invasion of the Body Snatchers all over again", and the Silvans would duplicate him and Benny, in order to replace them as well. While Lakis' aide Killian felt this was the only viable option, Lakis had no desire to replace the Doctor and become President of the Universe. (AUDIO: Truant)

In 2050, when Darius Pike saw his friends hypnotised, he decided that the Anubians replaced Gryffen, Starkey and Jorjie with robots, pod people, or clones. (TV: Curse of Anubis)

Behind the scenes
Both stories refer to the, or Body Snatchers, from the 1956 film Invasion of the Body Snatchers. These plant-like creatures — which come from actual pods — slowly replace each resident in the town of Santa Mira, including the protagonist in the film's final moments.

As revealed in the — and precisely like the Silvans — these pod people are nomadic parasites who have lost their home planet.