H is for... Hath

Piscine bipeds who accompanied human colonists from Earth to help colonise the planet Messaline in the year 6012. They breathed through special liquid filled devices when away from their natural habitat.
The mission commander was killed on arrival, and the resulting power vacuum led to an armed conflict breaking out between the humans and the Hath. Each side possessed Progenitor machines, which could reproduce their kind from a genetic sample. The war raged for only a few days, but casualties were such that by the time the Doctor, Donna and Martha arrived, everyone thought that it had been going on for much longer, as new generations of soldiers were created. Both armies had a goal - to find something known as "the Source", which they believed would give them victory. This turned out to be a terraforming device.
Martha was captured by Hath during a raid, becoming separated from the Doctor and Donna. She found that she was able to communicate with them. On discovering the location of the Source at the same time as the humans, Martha realised that the quickest way to it was over the inhospitable surface of the planet. A Hath named Peck agreed to accompany her, but he was killed when he fell into a pool whilst rescuing her and his breathing apparatus was smashed.
The Doctor activated the Source, and the planet began to change to a more hospitable environment, whilst the Hath, led by Gable, and the humans agreed to work together.
A pair of Hath were later seen by the Doctor in an alien bar where he went in search of Captain Jack prior to his regeneration, and other Hath frequented the Maldovarium when Colony Sarf went in search of the Twelfth Doctor on Davros' instructions.

Played by: Paul Kasey (Peck), Ruari Mears (Gable). Appearances: The Doctor's Daughter (2008).
Cameos: The End of Time Part II (2010), The Magician's Apprentice (2015).

• Peck and Gable were named after Hollywood legends Gregory Peck and Clark Gable.
• Peck's death is one of the stupidest in the history of the series. If you want to kill a fish-like creature, you don't have it drown...