Bats were nocturnal, flying mammals found on Earth. They were feared by some humans, who believed that they sucked the blood of others. However few species of bats actually did so. (AUDIO: The Caves of Erith)
Many vampire legends told of vampires turning themselves into bats and back into their true forms.
Encounters with bats
Whilst on the run from the Daleks, the First Doctor, Ian, Barbara and Vicki landed in a faux-haunted house. Built for the Festival of Ghana, the building housed replicas of classic fictional monsters like Dracula. The house was also infested with bats, groups of whom occasionally took flight so as to add to the general spooky environment of the "haunted house". (TV: "Journey into Terror")
During the time he travelled with John and Gillian, the First Doctor again encountered bats. Soon after landing on the Haunted Planet, bats attacked John. The Doctor successfully shooed them away with a big stick, but he surmised that the bats were being controlled by someone evil. (COMIC: The Haunted Planet)
Aukon used bats as servants on the vampire planet. He sent them to attack the Fourth Doctor and Romana II. The Doctor claimed they were harmless shortly before he was bitten. They travelled in a flock of many thousands. Later, Romana was also bitten by a bat while in a trance. (TV: State of Decay)
A kind of bat lived in the caves of Androzani Minor. The milk from the queen bat worked as an antidote to Spectrox toxaemia; the Fifth Doctor descended there in order to earn it. (TV: The Caves of Androzani)
Lebenswelt was home to red bats which constantly fought each other. When the colony on that planet began experiencing power failures, the bats began attacking the people, sensing that they had become more vulnerable. (PROSE: The Book of the Still)