Two disguised Roboforms.
The Roboform were cybernetic alien scavengers who accompanied invaders attacking planets and would take anything of value.
They have been described as "Pilot Fish" by the Doctor, groups of fish which follow sharks to help themselves.
In appearance they have robotic like faces but are usually disguised when scavenging a planet.
The Christmas Invasion[]
A group of Roboform's traveled alongside the Sycorax warriors who invaded Earth on Christmas day in 2006. They encountered the newly regenerated Tenth Doctor, disguised as music playing Santa Claus's they sent a robotic Christmas tree to the flat of the Doctor's companion Rose Tyler where the deadly tree was just narrowly defeated.
There musical instruments were actually disguised machine guns and flamethrowers.
After the Sycorax ship was destroyed along with the rest of the Sycorax warriors, the group of Roboform's survived but left abandoned on Earth.
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.
Another group of Roboforms joined the alliance against the 11th Doctor in order to save the universe.