Snake

Snakes were a variety of reptiles.

Earth snakes
A snake from Brazil was one of the animals on board the Ark. (TV: The Ark)

In Australia in 1987, Tegan Jovanka's father was hospitalised after a snake bite. (PROSE: Encounter on Burnt Snake Flat)

In Haiti in 1915, Charles Oscar Etienne was killed by a snake intended for the Doctor. (PROSE: White Darkness)

One snake which resided in Twycross Zoo was draped around Wanda's shoulders when she first met the Tenth Doctor and Rose Tyler; Rose was afraid of it. (COMIC: Untitled)

George Thompson believed that the noise that the lift made (PROSE: My Special Book) in the Rowbarton Estate, (PROSE: Night Terrors) was actually a dragon or a monster, living in the walls like the snake in Harry Potter. (PROSE: My Special Book)

Snakes on other planets
A type of giant snake was native to Metebelis III at some point in its history. (TV: The Green Death)

Giant purple Ice Snakes lived on Centuria. (COMIC: Enemy Mine)

The Church once fought lava snakes. (TV: The Time of Angels)

A giant species of snake called the Oroborus was able to consume time. (TV: Oroborus)

Another giant species of snake known as the Sythaks were native to the swamps of Palliox, a planet near Trion. In 1905, the travelling showman Thaddeus P. Winklemeyer had two Sythaks in his freakshow. (AUDIO: Freakshow)

In a jungle on an unnamed planet, Gillian was threatened by a hooded snake, but the First Doctor saved her by throwing his coat over it. (COMIC: The Didus Expedition)

As part of his magic act for the Gods of Ragnarok in the Dark Circus, the Seventh Doctor appeared to conjure a snake, then transform it into his umbrella. (TV: The Greatest Show in the Galaxy)

Colony Sarff was a creature comprised of many snakes working together to mimic one humanoid individual. (TV: The Magician's Apprentice / The Witch's Familiar)

Usages
On Thoros Beta, sand snakes were consumed by the Mentors, such as Kiv. (TV: Mindwarp)

On Earth snake skin was sometimes used to to make shoes. (PROSE: Eternity Weeps, Mad Dogs and Englishmen)

Emotive responses
Styre's experiments into fear on Sarah Jane Smith caused her to hallucinate a snake on her arm. (TV: The Sontaran Experiment) Polly Wright was also always afraid of snakes. (AUDIO: House of Cards)

When the Mara took physical form away from Dark Places of the Inside it took the form of a giant snake, both as a tattoo and as a physical snake. (TV: Kinda, Snakedance)

Clara Oswald confessed she hated snakes. (PROSE: Into the Nowhere)

Behind the scenes
While PROSE: My Special Book claims that there was a snake in Harry Potter, it was actually a basilisk; this, however, can be explained by George Thompson simply misremembering the story.