Golem

Golems were artificial humanoid lifeforms fashioned as soldiers. Golems were usually made of aluminium silicate clay, though sometimes were made from gold.

Characteristics
Once given an order, a golem will tirelessly work to that goal even if the order is rescinded by its master.

The Eighth Doctor refereed to golems as as unthinking killing machines and the best soldiers in the universe, due to their near invulnerability. (BFA: The Beast of Orlok)

History
Baron Teufel brought a pair of experimental of golems, known as Hans and Greta, to earth in the early 19th century. Unlike other golems, Hans and Greta were capable of learning and adapting. They also appeared human, appeared to age, and even believed themselves to be human.

Golems were outlawed by Article XII of the Galactic Code. (BFA: The Beast of Orlok)

The Valnaxi created a biologically modified species of golem as defenders of Valnaxi art. These golems took over any lifeform they come into contact with and turned that lifeform into a living statue of gold.

The golems, however, could only extend themselves to a certain number of lifeforms, so there were a limited number of them. Any life taken over by a golem immediately became another golem to become a guard. When fighting Wurms, however, golems were easily destroyed shot by the Wurms. The Golem's victims included a miner and a vulture who stumbled upon a golem in the 22nd century. (NSA: The Art of Destruction)

Behind the scenes
The name "Golem" comes from Jewish folklore, in which it is a man-like creature made from mud or clay.