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"Caber" was the nickname given to the Duke of Forgill's gamekeeper, so called because of his Highland Games prowess. His real name was Gaelic and difficult to pronounce.
Caber was abducted by the Zygons and impersonated by one of them. The false Caber shot Munro and Harry Sullivan, killing the former and wounding the latter. The Fourth Doctor found the real Caber in the Zygon ship, along with the other abducted humans, and helped them escape prior to setting the ship to self-destruct.