Nimon

The Nimons were a species of parasites on a galatic scale, using up civilizations and moving on.

Appearance
The Nimons were large black-skinned humanoids resembling the mythical minotaurs. Their horns can shoot out blasts of energy. Nimons drained energy direct from their victims, leaving them as husks. The Nimon have mighty, rumbling echoing voices.

Life cycle
The Nimons could use an artificial black hole as a gateways from planet to planet. This technology was effective but not energy efficient. Due to their high energy needs, the Nimons undertook migrations they called the Great Journey of Life. Usually, one Nimon was sent to a planet, representing themselves as a god with advanced technology where it would contstruct a maze-like Power Complex, a building-sized labyrinth of circuitry designed to generate a new black hole which it then used to transport more Nimons to the planet. The Nimons would then drain that world dry and move on.

History
When the Doctor and Romana stopped the Nimon they had used exhausted Crinoth and had moved on to Skonnos, home to the faded Skonnos Empire. The Nimon used the Skonnon Soldeed as their priest and demanded tribute of hymetusite crystals and demanded tribute from the planet Aneth. (DW: The Horns of Nimon)

The Doctor would later encounter the Nimons again on Earth. (BFA: Seasons of Fear)

Behind the Scenes
Anthony Read, the creator of the Nimons drew heavily from the ancient Greek myth of the minotaurs when creating the Nimons.

When making The Horns of Nimon production team had conceived the Nimon's large "heads" as masks that would open up, revealing their dessicated actual faces beneath. However, the budget could not stretch that far and so they never attempted to show that the Nimons had a hidden true form.