Roboform

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

Characteristics
The Roboforms were humanoid robots with featureless, vaguely skull-like heads. They were typically seen wearing dark cloaks which concealed most of their bodies, only the face being visible. (DW: The Runaway Bride) When on Earth during Christmas, they tracked the Doctor there as they could "smell" regeneration energy. They wore Santa Claus suits and masks to blend in (giving them the nickname of Robot Santas). (DW: The Christmas Invasion, The Runaway Bride) They also had a variety of concealed weapons, such as projectile weapons disguised as musical instruments, a remotely controlled Christmas tree (DW: The Christmas Invasion) and bombs disguised as Christmas ornaments (DW: The Runaway Bride). They also used basic machine guns when controlled by the Empress of the Racnoss. (DW: The Runaway Bride)

History
In 102 A.D., 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. (DW: The Pandorica Opens)

In 2006, the Roboforms were travelling with the Sycorax when they were attracted by the energy being released by the Doctor. They planned to kidnap him and use the 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. (DW: The Christmas Invasion)

In 2007, the Empress of the Racnoss took control of the Roboforms, using 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 Doctor was able to destroy them using a sonic pulse). They also defended the Empress herself. (DW: The Runaway Bride)