Physically, Monoids were about the same height as humans, but their skin was dark and scaly. They all had mop-top hair, which varied in darkness. Their feet were like flippers, which probably contributed to their slow movement. The most striking feature of the Monoids, though, was the single eye in the centre of their heads. (TV: The Ark)
While in the 27th century they still possessed the power of speech and had individual names, (AUDIO: The Kingdom of the Blind) by the 57th Segment of Time they had no visible mouths, relying on sign language for communication, although they apparently ate and drank through a hole in the side of their necks. Instead of names, the Monoids of this era had numbers based on rank. (TV: The Ark)
At some point in history, a lost Halyvon ship crashed on the Monoid homeworld. It was the Halyvons that gave the one-eyed race the name "Monoids". The crash was a result of in-fighting among the Halyvons, and while one faction tried to conquer and enslave the Monoids, a survivor of the other faction destroyed the telepathic boosters and without the ability to communicate telepathically they became the slaves of the Monoids. The Monoids did not allow their slaves to use speech or individual names, and cut out their tongues as punishment. (AUDIO: The Kingdom of the Blind)
Bernice Summerfield and Jason Kane encountered Monoids in the 27th century. The Halyvon slaves of these Monoids re-activated the ancient Halyvon machines and re-acquired their telepathic link. They rebelled against their masters and used their psionic powers to seal the Monoids' mouths and take away their speech. (AUDIO: The Kingdom of the Blind)
The Monoids arrived on Earth long before the Ark went into space. Their own planet was dying, and they offered their services in return for a place on the Ark. The humans accepted and welcomed them on the trip. While travelling, the Monoids fulfilled a near-slave role, carrying out menial tasks for the humans.
After several hundred years of travelling, the Ark neared its destination, Refusis II. When the Doctor, Steven and Dodo visited the ship (christened "the Ark" by Dodo), they brought with them a strain of the common cold virus, to which neither the Guardians nor the Monoids had any resistance. The virus spread rapidly and killed several Guardians and Monoids before the Doctor was able to devise a cure. Thinking all was well, the TARDIS crew left — but a mutation of the virus appeared, and apparently was more virulent to the human population than the Monoids. The fever sapped the humans' will, and the Monoids saw their opportunity to seize control of the Ark and end centuries of servitude. The Guardians had encouraged research into voice boxes for the Monoids, and had also developed heat prods which the Monoids could use as weapons. With these advantages, and the fact that the Guardians were unprepared for conflict, the Monoids managed to kill many of them and make prisoners of the rest, using heat prods to keep their captives in order. The Monoids completed the humans' statue by adorning it with a Monoid head instead of the intended human one. In this, they planted a nuclear bomb to be detonated when they abandoned the Ark and left the humans behind.
After Monoid Two did not complete communications from the surface of Refusis II, the Monoids began a full-scale landing. However, mutiny resulted in the deaths of several Monoids as fighting ensued. The bomb was ejected into space by the humans, with help from a Refusian, saving the Ark. The surviving Monoids returned aboard. The Refusians welcomed both species to their planet if they were to become friends. Both agreed. (TV: The Ark)
In the video game Happy Deathday, played by Izzy Sinclair on the Time-Space Visualiser, a Monoid was among a host of "every single enemy" that the Doctor had ever defeated, who were assembled by the Beige Guardian and pitted against the Doctor's first eight incarnations. (COMIC: Happy Deathday)
The Sixth Doctor discovered Peri's mind was being controlled when he asked her if she remembered fighting the Monoids, which she did. He later admitted he and Peri had never fought the Monoids. (AUDIO: The Widow's Assassin)