Species 29s were originally a humanoid race with purple skin and short, white hair which fell down across the back of their heads. Later, they were transformed into incorporeal life-forms by the Ageless. They were moved temporarily into the bodies of cats. Even while in these bodies, they retained their basic abilities, such as intelligence and mind reading. (PROSE: Cats and Dogs)
In their humanoid forms, Species 29 had a technologically advanced home-world, with large building made from an unknown black material and a green lighting. They also had battleships with large laser cannons mounted on top. (PROSE: Cats and Dogs)
Species 29 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 7. The war raged on for thousands of years, and at one point 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 29s 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 also present at the trial, as an additional party. The Doctor deliberately caused a transdimensional quirk which gave the species 29s real bodies, in the form of cats. The Species 29s attempted to go to war against the Species 7s — who had been transformed into dogs — and the Doctor advised the Ageless to not give the Species 29s their real forms back. (PROSE: Cats and Dogs)