Squirrel

Squirrels were typically small rodents, easily identified by their outsized, bushy tails that were almost as long as their bodies.

Encounters with squirrels
When the Demon Magician threw up a huge wall around the TARDIS, the First Doctor used his "magic box" to super-size a squirrel. The Doctor, John and Gillian crossed the wall on the giant squirrel's back. (COMIC: A Christmas Story)

Slake once made some squirrels vampiric and used them to "guard" the Orpheum Theatre, the lair of Slake's vampire clan. When the Eighth Doctor and Sam entered the theatre, they were viciously attacked by the vampire squirrels. (PROSE: Vampire Science)

Squirrels lived in the forests of the Catskill Mountains. They were mostly gone when the Butler Institute got rid of the trees to build a project site, although Brodie saw one after a failed attempt to smash a security camera. They seemed to be returning after the site's destruction. (PROSE: Cat's Cradle: Warhead)

In dreams
In a dream induced by an entity controlling the Ninth Doctor's body, the Doctor and the life form wearing his body overfed a cartoon squirrel to the point where the squirrel's head burst off. (COMIC: The Cruel Sea)

Artificial squirrels
The Twelfth Doctor once dismantled his radio and used the parts to build a clockwork squirrel. (TV: Under the Lake) Whilst explaining the Bootstrap Paradox, the Doctor sat the clockwork squirrel above his amplifier. (TV: Before the Flood)

Breeds
Amongst the known breeds of squirrel were black squirrels. (AUDIO: Mother Russia)

Behind the scenes

 * In an interview with US chat show host Conan O'Brien in September 2015, Jenna Coleman revealed she had taken the clockwork squirrel prop from Before the Flood as a souvenir of her time on Doctor Who.