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 cream-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 Doctor there as they could "smell" regeneration energy. They wore bullet-proof Santa Claus suits and metal masks to blend in (giving them the nickname of Robot Santas). (TV: The Christmas Invasion, The Runaway Bride) They also played music in public with instruments such as trombones and tubas which were ostensibly used as flamethrowers and guns or rocket launchers. Remotely controlled Christmas tree (TV: The Christmas Invasion) and bombs disguised as Christmas ornaments. (TV: The Runaway Bride) They also 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. They attacked Rose Tyler and Mickey Smith and sent a robotic Christmas tree to their home. The Roboforms were driven off when the Doctor awoke. (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 (though the Tenth Doctor was able to destroy them using a sonic pulse). They also defended the Empress herself. The Empress's personal guard was disabled by the Doctor 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 where children were nested. (TV: The Runaway Bride)

Behind the scenes
Robot Santas are used as enemies in the Doctor Who: Legacy mobile game.