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The Hath appeared to look like a race of fish-like humanoids.
Hath were also a humanoid race with purple and orange, they had fish-like skin and small headcrests running along the top of their heads. They were slightly taller than most humanoids. They required nutrient liquid for breathing, this affected communication. When in a human-breathable atmosphere, they used a breathing structure attached to their mouths. (The Doctor’s Daughter)
They had possessed Progenation machines and generation chambers like the humans with whom they had been allied. They used the machines for the same purpose as their enemies, to create new soldiers.
The Hath also used heavy duty blaster rifles that fired barrages of bullets and long plumes of flames from the guns’ barrels. They had the same holographic mapping system as the humans, and when the Tenth Doctoruncovered a suppressed layer of information, it showed up on the Hath map as well. (The Doctor’s Daughter)
In universal history, the Hath joined Rassilon in the Alliance of Races against the Hyperion. After the Hyperion were defeated, the Alliance then went on to purge the universe of other threats to universal harmony, one of those threats being Count D’if of the Cybock Imperium. (Gangland)
The Hath left their planet by themselves, but later joined up with other species like humanity to share technology. They helped mine the ores needed to create the first colonies. (The Doctor’s Daughter)
The Hath were one of the 11 creatures who appeared in the hologram projection shown by the Atraxi as they were scanning Earth’s history to check if it was protected. (The Eleventh Hour)
According to one source, the Hath were also present on Messaline in 2014, almost 4, 000 years before their known colonisation of the planet. Upon landing The TARDISthere, Rose had found a Hath head on the planet and dragged it back to the ship. (A Rose by Any Other Name)
The Hath had also taken part in the colonisation of Messaline in July 6012 of the New Byzantine calendar. With the death of the mission commander, there was a power vacuum and the species fought to take the planet for their own.
After a seven-day battle, the two races found the Source, an artefact that mythology told them was the breath of the Great One. the Tenth Doctor, arriving before the human or Hath armies, broke the Source and terraforming gases within escaped. The two races ended their conflict and agreed on peace after the murder of Jenny by Cobb, the leader of the human forces. (The Doctor’s Daughter)
BEHIND THE SCENES
Russell T Davies’ original idea for the Shadow Proclamation scene in The Stolen Earth included an appearance by several Hath, but his plans to feature a “rogues gallery” of alien races in the scene fell through; ultimately the Judoon were the only returning race featured.
Prior to the broadcast of The Doctor’s Daughter, it was rumoured that the Hath had the appearance of red bubbles and communicated through expanding and letting out smaller bubbles.
While bubbles had not appeared to be the Hath’s only form of communication, Doctor Who Confidential showed that lines were actually scripted for the Hath (even if they only groaned throughout the episode).
It has been suggested by the director of The Doctor’s Daughter, Alice Troughton, that the fish seen in the Source Chamber are the ancestors of the Hath.