Tryod

s were a species which the second incarnation of the Doctor encountered at two far-removed points in their evolutionary history.

Biology
Tryods were nine feet tall and had leathery bodies with fine, golden hair which grew most verdantly in the centre of their foreheads, their backs, and their legs. They had very long teeth and claws and leathery tails.

On the back of their necks was an obvious protrusion. It was an organ which secreted a healing fluid when they were injured. They could only be killed by damaging this organ, which otherwise could resuscitate them, no matter what the injury.

Even during the Doctor's first encounter with them, when they were considerably less-evolved, Tryods were capable of showing mercy. Though they apparently could not talk at this juncture, they did display the ability to rise above a primitive killing instinct. The Doctor witnessed one refuse to bite into another's healing organ, despite having been victorious in battle. When the Doctor revisited them, thousands of years later, they were fully able to talk.

They were known to eat at least some types of lichen found in caves.

History
When the Second Doctor landed on the ostensible homeworld of the Tryod, they were under threat by the dominant humanoid species on the planet. Not only were their hides commercially valuable, the humanoid species' leader, Qar, was seeking permission from the galactic leader, or Raymah, to massacre them. They were viewed as vermin in need of extermination, as the land they lived on was replete with mineral wealth.

With the help of Asiries, however, the Doctor was able to avert their destruction. In fact, thousands of years after their near extinction, they evolved into the dominant species on the planet, while the humanoids had vanished.