Triceratops

Triceratops was a type of herbivorous dinosaur on prehistoric Earth and the biggest of the ceratopsians.

Biology
Triceratops had two large horns at the top of their heads and one small one near their nose. They weighed over a ton and could be very dangerous in a fight. They had a frill made of solid bone above their heads and ate with a beak.

Encounters
Triceratops were among the dinosaurs transported to the 1970s by Operation Golden Age. The Third Doctor encountered a few of these Triceratops. (TV: Invasion of the Dinosaurs)

In an alternate universe, Triceratops were one of the prehistoric species bought back from extinction by the Silurians. Triceratops were allowed to roam free (PROSE: Blood Heat)

The Eighth Doctor and Samantha Jones once returned a baby Triceratops to its mother. (PROSE: Unnatural History)

The Triceratops was worth 550 points in the I-Spyder Book of Earth Creatures. (PROSE: The Last Dodo)

Triceratops was among the species placed aboard the Silurian Ark to repopulate the Earth with after the catastrophe the Silurians incorrectly predicted had passed. In the 24th century, when the Eleventh Doctor and his companions came aboard the Ark, they used a Triceratops, nicknamed Tricey as a mount. Solomon later had the Triceratops killed to make an example out of it.

Another Triceratops was seen on Brian's postcard's picture of Siluria, indicating that the Ark had at least one other Triceratops specimen besides Tricey which was taken to Siluria by the Eleventh Doctor after he stopped the Ark from returning to Earth. (TV: Dinosaurs on a Spaceship)

Similar creatures
The planet Skaro was also home to creatures that resembled dinosaurs, specifically the Triceratops, but with only one horn. Normally peaceful herbivores, they would attack Daleks on sight. The Third Doctor and Finney used a herd of these to destroy a group of Daleks that had taken them to Skaro. (COMIC: The Planet of the Daleks)

Behind the scenes

 * According to non-narrative material in DWBIT 7, the Triceratops was also native to the planet Raxas, and along with the other Raxas dinosaurs, proved to be more than a match for a Dalek attack.