The Arucha were an alien species which stole items of practical value.
The Arucha had battleships which were a gun metal grey. The ships had dark corridors and computers which could listen in and watch conversations on planets below and detect space and time machines. They also had landing crafts which had retro rockets.
They dressed in what were described as combat outfits. They carried blasters, sonic lances, and translation devices. They also carried a sensor device that perceived the colour blue for them. (WC: The War of Art)
The Arucha evolved on the same planet as several other sapient species. This led to fierce and violent competition, with the Arucha eventually winning and developing their acquisitive nature.
At some point after the 32nd century, a ship of Arucha arrived at the Paloma Centre, tracking the Eleventh Doctor's ship and planning to steal whatever he was looking for. The Doctor tried to avoid them, hiding in the Blue Room and then sending the android copies of the artists to attack them. Eventually, he was able to convince them that he was looking for The Physical Impossibility of Getting a Shanghorn into a Glass Tank and that it was a weapon. The Arucha stole it and escaped, but only found out it was simply art two years later. (WC: The War of Art)