Clanger

A species of mice-looking knitted pig-pink characters was featured in a puppet TV show, and lived on a blue Moon-like planet. They communicated by making whistling noises.

watched an episode of Clangers while he was imprisoned on Fortress Island. He whistled along with the noises they made. When the prison governor Colonel George Trenchard asked him what he was watching, the Master responded that "it seems to be a rather interesting extraterrestrial lifeform." Trenchard told him it was just puppets for children. The Master expressed disappointment over this, (TV: The Sea Devils) and was unconvinced; he made a note to investigate whether he could find the planet of the Clangers and forge an alliance for world domination with them. (PROSE: The Secret Diary of the Master)

Information from invalid sources
A member of the species once stood in the TARDIS while dressed as the Fourth Doctor. (NOTVALID: Doctor Who DoppelClangers)

Other matters

 * The Clangers scene was a last-minute addition, as episode one of The Sea Devils was 90 seconds short of the typical run time.
 * The episode the Master watches is The Rock Collector which was first broadcast on 25 April 1971.
 * The Clangers scene would later be echoed by enjoying Teletubbies in the television story The Sound of Drums.