Species 7s were originally a humanoid race with purple skin and short, white hair which fell down across the back of their heads. Later, they were turned into incorporeal life-forms by the Ageless. They were temporarily moved into the bodies of dogs. Even while in these bodies, they retained their abilities such as intelligence and mind reading. (PROSE: Cats and Dogs)
In their human forms, Species 7 had a technologically advanced home world, with green lighting and large building made from an unknown black material. They also had battleships with large laser cannons mounted on top. (PROSE: Cats and Dogs)
Species 7 were originally a humanoid species who lived on a planet where everything was extremely organised. At some point in their history, they went to war against a fellow humanoid race, the species 29s. The war raged for thousands of years and threatened to destroy the entire universe.
Eventually, a neutral race known as the Ageless intervened, using their superior technology to remove the minds of the species 7s from their bodies. They were kept in this incorporeal state for a thousand years, after which the Ageless decided to review their punishment with a trial. The Tenth Doctor was present at the trial as an additional party. The Doctor deliberately caused a transdimensional quirk which gave the species 7s real bodies: dogs. The Species 7s attempted to go to war against the Species 29s — who had been transformed into cats — and the Doctor advised the Ageless to not give the Species 7s their real forms back. (PROSE: Cats and Dogs)