The Cold (Time and Relative)

The Cold was an entity that existed on Earth at least as far back as the First Great Ice Age. The First Doctor encountered it in London shortly after leaving Gallifrey with Susan. He was unsure whether it was a formless intelligence that could manipulate frozen water, or existed within ice itself. The Cold had lain dormant for millennia, and revived around 1963. It planned to rid the world of what it considered a mammalian infestation. The Doctor, still bound by the Time Lord non-interference policy, was going to allow it to do so. However, Susan convinced him to stop it. The Doctor used the TARDIS to reach into other dimensions and condense the Cold into a single snowflake. After several failed attempts, he managed to reach the far future and leave it on Pluto. (PROSE: Time and Relative)

Behind the scenes
The Cold later appeared in Kim Newman's short story Cold Snap.