Solonian

The Solonians were a race with several different forms.

Biology
As their planet, Solos, went through its seasons, the Solonians gradually mutated from one form to the next form. This naturally occurred with radiation and a special green crystal from the planet. These changes could also be artificially caused by pollution.

Humanoid Solonians
The humanoid Solonians were the form which existed during the spring of Solos. They appear to be dark-haired humans. The main difference from humans was their ability to breathe the poisonous mists of Solos. They were at a low level of technology when they were discovered, with a simple farming and hunting culture. (TV: The Mutants)

Insectoid Solonians
The Insectoid Solonians (pejoratively known as Mutts to the human overlords) were the intermediate form from the start of summer on Solos (something like a mobile pupa). These Mutts were a bit shorter than humans and covered in a thick black exoskeleton.

They had a head, thorax and abdomen, as well as clawed arms and legs. They had bulging yellow eyes, a wide opening for a mouth and long pincers beside their mouths. Insectoids seemed to have decreased intelligence and could not speak, though this was likely just a side effect of the transformation.

Near the beginning of summer, the humanoid Solonians would be instinctively compelled to seek out the necessary radiation and crystals for the transformation.

Though the mutation would occur naturally from the radiation and crystals from certain caves, the changes could be brought on early with pollution. Unnatural changes could leave the victims confused and uncertain about the entire process. (TV: The Mutants)

Advanced Solonians
The Advanced Solonians were the form which existed during the summer of Solos. The insectoid Solonians were transformed into advanced Solonians by more radiation. Though they had the basic shape of a human, advanced Solonians were almost angelic, with a robed body and scintillating colours. They communicated telepathically and had a wide variety of powers, including the ability to pass through solid matter and to shoot energy beams. (TV: The Mutants)

Culture
The Solonians used engraved images in their writing. The Third Doctor was able to decipher engravings depicting the four seasons of Solos.

Each of these seasons lasted 500 years as Solos only circled its sun once every 2000 years. These seasons triggered the mutations, but the Solonians were unaware of this due to their own cryptic writings and long periods under mutation.

Solonians were often segregated from humans. They were forced to use different facilities such as a separate transmat. (TV: The Mutants)

History
Sometime in the 25th century, the Earth Empire discovered Solos and started to exploit it. The Solonians were taken advantage of and their planet was stripped of its resources.

By the 30th century, tensions were rising and the mutations from the modified atmosphere were starting to increase. Solos was to have independence but the Marshal had the administrator assassinated and placed Skybase One, and hence Solos, under martial law.

Ky killed the marshal and a number of humans stayed behind to help transition the planet back to its former functionality. (TV: The Mutants)

Behind the scenes
A Mutant costume was cosmetically altered and reused to represent a generic insectoid alien for the opening scenes of The Brain of Morbius, but the production team did not intend to suggest they were the same species any more than in the case of the Axons and the Krynoid (another case of a recycled costume). The novelisation of the story makes it clear that the creature belongs to an entirely different species. Despite this, the Doctor does call the creature in The Brain of Morbius a Mutt, like the insectoid Solonians were called, and acts like he recognises the creature. Ohica also mentions that the last ship to arrive on Karn before the Doctor was a "cruiser of the Mutts".