D is for... Dinosaurs

A diverse range of reptilian species which dominated life on Earth for millions of years, dying out almost overnight around 65 million years ago.
The Doctor was hoping to encounter some when the TARDIS arrived on the island of Atlantis in the late 20th Century. He would not meet any until he was into his third incarnation and in exile on Earth, however.
UNIT was called in to investigate strange events at a scientific research centre built into a cave system in Derbyshire. Two of the centre's scientists had gone pot-holing and were attacked by a large savage reptile. One was killed, and the other driven insane.

When the Doctor investigated the scene of the incident he was also attacked, and recognised the creature as a form of dinosaur - its species unknown to him. It was prevented from killing him when it was called away by a sonic signal. The Doctor deduced that it was some kind of guard, and that there were two different reptile species living in the caves - the dinosaur and a smaller bipedal race who had some advanced science. These latter were the Silurians, who had once been the dominant life-form on the planet. They had gone into hibernation millions of years ago to avoid a catastrophe when a small planetoid threatened to tear away the atmosphere as it passed close to the Earth.
The Silurians were forced to return to hibernation when the research centre appeared to be about to explode, and presumably their dinosaur guard would have gone into suspended animation with them.

After his exile had been lifted, the Doctor and companion Jo Grant found themselves on a ship in the middle of the Indian Ocean. This was the SS Bernice, which had famously vanished in 1926. They witnessed the vessel come under attack from a Plesiosaur. The ship was actually being held as an exhibit in a machine on an alien planet. It is not known if the dinosaur had been picked up from a different point in time and placed into the same exhibit, or if it really had attacked the ship in 1926 and the two had been scooped up together.

Returning to the 20th Century from Medieval England with new companion Sarah Jane Smith, the Doctor found London to be deserted. Investigating a looters' lair, they were attacked by a Pterodactyl. Fleeing arrest by the army, who had placed the city under martial law, they then encountered a Tyrannosaurus Rex. Reunited with UNIT, the Doctor and Sarah learned that a number of dinosaur species had been appearing and disappearing throughout Central London over the last few weeks. Other species included the Stegosaurus, Triceratops and the Apatosaurus (described by Sgt Benton as a Brontosaurus).

These creatures were being brought to the 20th Century by a scientist named Whitaker, as part of a plan to evacuate London in order that a more audacious scheme could be carried out unhindered. Whitaker was part of a group who intended to roll back time to a perceived golden age, free of technology and pollution. Only a few hand-picked individuals would be permitted to accompany them - people who had been duped into thinking they were going to a new planet.
The Doctor attempted to capture one of the dinosaurs so that he could identify where the time travel technology was operating from - only for his efforts to be hampered by Captain Mike Yates, who was a member of the group. Sarah was almost killed when a captive Tyrannosaurus woke up, its chains having been sabotaged.
The Doctor put a stop to the plan, but Whitaker and the group's leader - MP Sir Charles Grover - were sent back to an unspecified time in history.

The next two dinosaur creatures encountered by the Doctor were actually genetically engineered. The Skarasen was not a native species at all, originating on the Zygon homeworld, whilst the Myrka was an unknown aquatic reptile, adapted for use as a weapon by the Sea Devils. It thrived in the deepest oceans and was susceptible to ultra-violet light. Its body carried a powerful electric charge which could kill a human.

The Rani admired the dinosaurs, and had her Time Manipulator scheme worked on the planet Lakertya she might have returned the Earth to the time of the creatures to see how they would have evolved had they not been wiped out. Earlier, she had kept a number of Tyrannosaurus embryos in glass jars in the console room of her TARDIS. When the Doctor sabotaged the ship, sending it hurtling into the future out of control, Time Spillage occurred, and the embryos began to rapidly grow - threatening her and the Master.

When River Song failed to kill the Doctor at Lake Silencio, it created an alternative world in which all of history existed at the same moment in time. Pterodactyls flew over the streets of London, nesting in the parks. People were advised not to feed them as they were regarded as little more than flying vermin.

In the year 2367, a large spaceship was found to be on a collision course with the Earth. The Indian Space Agency called on the Doctor for help. He put together a team of friends, including companions Amy and Rory Williams, and went to investigate. The ship proved to be an Ark, launched into space by the Silurians at the same time that they had gone into hibernation on Earth. It was now on its automated return journey - its Silurian crew having been murdered by the trader Solomon.

On board the ship were a number of different dinosaur species, including Ankylosaurus, Triceratops, Pteranodons and Velociraptors. A Triceratops became quite friendly with Rory's dad, Brian, even playing fetch with him. The Doctor, Rory and Brian rode on the creature's back to escape from Solomon's robot servants. It was later killed by one of the robots on Solomon's orders. The Doctor managed to stop the authorities from destroying the Ark with a missile strike and sent the spaceship off to a new uninhabited planet where the dinosaurs could live, which he named "Siluria".

Following his next regeneration, the Doctor was unable to control the TARDIS and it crash-landed on prehistoric Earth. The ship was swallowed by a Tyrannosaurus Rex, which was then brought forward in time to the late Victorian era when the ship dematerialised whilst still inside it. The Silurian detective Madam Vastra knew how to placate the creature, which was confined to the Thames by the Houses of Parliament. The Doctor planned on taking the dinosaur home, but it was killed by the Half-Face Man - a clockwork 'droid who required a piece of its optic nerve to repair his own systems. The Doctor deduced that the 'droids must have crashed on Earth at the time of the dinosaurs for them to have known this would be compatible.

During his fifth incarnation, the Doctor and his companions found themselves in a cave system full of dinosaur fossils. He told Tegan and Nyssa of the theories surrounding how the creatures came to be extinct, including the impact of a massive asteroid onto the Earth's surface. He stated that he had always meant to travel back and find out for himself. Subsequent events would allow him to witness what had really happened. It was not an asteroid which hit the Earth, but a space freighter from the 26th Century whose guidance systems had been overridden by the Cybermen. His young companion Adric had attempted to disconnect this device, causing the ship to hurtle back through time some 65 million years. Adric perished in the collision which led to the extinction of the dinosaurs, and ultimately to the evolution of the human race.

Appearances: The Silurians (1970), Carnival of Monsters (1973), Invasion of the Dinosaurs (1974), Earthshock (1982), Warriors of the Deep (1984), Mark of the Rani (1985), The Wedding of River Song (2011), Dinosaurs on a Spaceship (2012), Deep Breath (2014).