Roboform

Roboforms were scavengers who travelled (as the Tenth Doctor said, like pilot fish) alongside invaders who took anything on the planet of value to them before the main threat (the metaphorical shark) arrived.

Biology
The Roboforms were robots with featureless, vaguely skull-like heads that were gold-coloured. They were typically seen wearing full-length black cloaks which concealed most of their bodies, with only the robotic faces visible. (TV: The Runaway Bride) When on Earth during Christmas, they tracked the Tenth Doctor there, as they could "smell" regeneration energy. They wore bullet-proof Santa Claus suits and metal masks to blend in. (TV: The Christmas Invasion, The Runaway Bride)

They also played music in public with brass instruments such as trumpets, trombones and tubas, which were ostensibly used as machine guns, flamethrowers, and rocket launchers respectively. They also use remotely controlled Christmas trees (TV: The Christmas Invasion) and bombs disguised as Christmas ornaments. They used basic machine guns when controlled by the Empress of the Racnoss. (TV: The Runaway Bride)

History
In 102, the Roboforms were among the groups who joined the Alliance. They came to Stonehenge and helped imprison the Eleventh Doctor in the Pandorica in order to save the universe. (TV: The Pandorica Opens)

In 2006, the Roboforms were travelling with the Sycorax when they were attracted by the energy being released by the Tenth Doctor. They planned to kidnap him and use his energy to power their ships. One group attacked Rose Tyler and Mickey Smith when they were Christmas shopping, but their targets escaped and their missile launcher operator inadvertently felled a Christmas tree that killed them. Another group sent a robotic Christmas tree to Jackie Tyler's apartment at the Powell Estate in another attempt to kill them, but the Doctor blew the tree up with his sonic screwdriver. The Roboforms were then driven off Earth by the Doctor. (TV: The Christmas Invasion)

In 2007, the Empress of the Racnoss took control of the Roboforms. She used them to keep track of Donna Noble. They attempted to kidnap her after she vanished from her wedding, then attacked her wedding reception with exploding Christmas tree baubles, though the Tenth Doctor was able to destroy them using a sonic pulse by firing his sonic screwdriver through the reception DJ's amplifiers. More Roboforms also defended the Empress herself. The Empress's personal guard, described by the Empress as "such good shots", was disabled by the Doctor, who had disguised himself as a Roboform to infiltrate the Empress' lair, when he produced a remote control from his dimensionally transcendental pocket and made the Roboforms "relax". They all slumped over and remained motionless, even as the River Thames flooded the laser drilling site to the core of the Earth, where the Racnoss children were located. (TV: The Runaway Bride)

Behind the scenes

 * Robot Santas are used as enemies in the Doctor Who: Legacy mobile game.
 * In their first appearance in The Christmas Invasion, the Roboforms wore ornate metal masks with mesh eye-holes and separate long false beards. For their return in The Runaway Bride, the masks were redesigned, now being a single large piece of fibreglass, with black-and-white eye-holes (although, in some scenes where Donna is kidnapped by a taxi driver Roboform, large grey eye-holes can be seen). The reason for the mask redesign was because of the new Roboform heads (which were also made of fibreglass), and the masks from The Christmas Invasion were insufficient for covering the new heads.