A is for... Aggedor

The Royal Beast of the planet Peladon. These savage creatures once lived on the wild Mount Megeshra, and were hunted to extinction by the nobility of that world as a rite of passage. Their fur would trim regal garments, and their horns adorn helmets. Over time they gained a mythical status, and one of them - Aggedor - became the planet's religious symbol. A temple to Aggedor was set up in the Royal citadel, replete with a huge stone statue of the beast. At the time that Peladon was petitioning to join the Galactic Federation, the High Priest Hepesh found that one of the creatures had survived. He kept it hidden in the secret tunnels beneath the citadel and trained it to do his bidding. Hepesh did not want his world to join the Federation, and he determined to use Aggedor to further his aims. He began by using it to kill the pro-Federation Chancellor Torbis - his own brother.
Aggedor's temple was so sacred that when the Doctor found his way into it by accident he was told that he was sentenced to death for blasphemy. This was after Hepesh had used the King's Champion, Grun, to lure the Doctor into a trap where he was to have been killed by the beast. The Doctor was able to placate it using a form of hypnosis, plus an old Venusian lullaby - the first line of which was "Close your eyes my darling, well three of them at least...". In the original Venusian it ran "Klokleda partha menin klatch, haroon, haroon, haroon...". The tune is similar to "God rest ye merry gentlemen".
When Hepesh's forces tried to mount a coup, the Doctor brought Aggedor to the throne room to show that the beast was real and no legend. Hepesh ordered it to kill the Doctor, but he had been cruel to it and so it killed the High Priest instead.
Fifty years later, the Doctor returned to Peladon. The spirit of Aggedor was once more causing death and destruction throughout the land. This was seen as a sign of disfavour against the Federation. Aggedor's image would appear from nowhere and strike down anyone who collaborated with the aliens.
Entering the temple of Aggedor was no longer worthy of a death sentence. The Doctor and Sarah Jane Smith were thrown into a pit beneath the temple by High Priest (and Chancellor) Ortron to face Aggedor's judgement. The Doctor used the same hypnosis technique (and song) to once again quieten the creature, which remembered him.
The Doctor later learned that the Ice Warriors, and a corrupt Earth engineer named Eckersley, were using a heat ray weapon coupled with the statue of Aggedor from the temple to cause the deaths.
After the Ice Warriors had been defeated, Eckersley abducted the Queen, Thalira, and tried to escape to a shuttle craft with her as hostage. The Doctor freed Aggedor from its pit and used it to hunt them. The creature killed Eckersley, but he managed to fatally wound it as he died.
It must be assumed that the beasts are now totally extinct.

Played by Nick Hobbs. Appearances: The Curse of Peladon (1972) and The Monster of Peladon (1974).