Caligula

Caligula, an emperor of the Roman Empire during the 1st century, was one of the individuals that Momus the Wise pulled out of time to help lead the people of Planet 12 of the Ridion Alliance into a new age of enlightenment. The Tenth Doctor noted that he wasn't as bad as he was portrayed in I, Claudius. (PROSE: The Lonely Computer)

In the year 37, the Daleks took over Caligula with mind control. Planning to harness his power to become masters of the Earth and influence all of human history, the mind control turned him violent and blood-thirsty. However, it also had the unplanned effect of destroying his sanity. Acting outside of Dalek behavioural conditioning, Caligula ordered his army to attack the ocean waves and made his horse, Inciatus, consul. When the Daleks tried to reassert control, Caligula refused to follow their orders and declared himself a living god. Because of the imbalance they inadvertently created, the Daleks abandoned the mission and returned to Skaro, reasoning that there were other nexus points in human history to manipulate instead. (COMIC: Empire of the Daleks)

The male version of Iris Wildthyme (who was in actuality a simulation created by her TARDIS) and "his" companion Panda attended a party thrown by Caligula. However, Panda claimed that the real Iris would never be associated with a madman like him, no matter how good the party was. (AUDIO: The Two Irises)

As the Fourth Doctor later recalled, Caligula once tried to throw the Doctor to the lions as he did not like the cut of his trousers. (AUDIO: The Labyrinth of Buda Castle)

Lucie Miller encountered Caligula during her travels with the Monk. She later told the Eighth Doctor that he was "well bonkers" and claimed that the Sensorites were "not as much of a laugh." (AUDIO: The Resurrection of Mars)

Coyn reminded the Eighth Doctor of Caligula, as both leaders used foreigners as personal guards. (PROSE: The People's Temple)

Behind the scenes
Caligula was portrayed by John Hurt in the BBC's 1976 adaptation of I, Claudius, by John Simm in the 2004 miniseries Imperium Nerone and by Samuel Barnett in the BBC's 2010 adaptation of I, Claudius.