Mr Sin

The Peking homunculus, also known as Mr. Sin, was a 51st century cyborg made in Peking for the commissioner of the Icelandic Alliance. It was made as a toy, a plaything for his children. It contained a series of magnetic fields on a printed circuit and a small computer, and a single organic component; the cerebral cortex of a pig.

It played a key role in history when the pig part took over and almost caused World War VI. After that it disappeared. It later turned out to be in the possesion of Magnus Greel, a fugitive war criminal from the fallen Supreme Alliance, who took it back in time with him, to the 19th century, and used it to do his bidding. (DW: The Talons of Weng-Chiang)

During the final battle between the Doctor and Magnus Greel, Mr. Sin became trigger happy and started firing at anyone who moves, including some of Greel's own men. During the battle Greel tried to use the time cabinet which The Doctor greatly advised against, warning him that it would cause a huge implosion, with everyone in the room in the centre of it. Mr. Sin heard it and started firing at Greel, it is unknown whether or not it was to save his own life, or because he became truly trigger happy.