Tryod

s were once encountered by the second incarnation of the Doctor when he was once apparently travelling on his own.

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.

Tryods were capable of showing mercy, as the Doctor witnessed one refuse to bite into another's healing organ, despite having been victorious in battle.

Tryod hide was valuable to the near-Human species on the planet.

History
to be added