Mechanoid

Mechanoids were large spherical robots originally built by humans in order to help colonize worlds until they were abandoned and forgotten on Mechanus. The Mechanoids became a force in their own right and Mechanus became their home planet. They were the enemies of the Daleks.

History
The Mechanoids first encountered the Daleks when they built a space station for refueling on the route to the planet Oric. The Mechanoids used their suspicion ray on one of the Daleks there, causing it to perceive other Daleks as its enemies and kill them. The Mechanoid Interceptor which held the ray was destroyed by the Daleks, but the Dalek ship was then attacked and melted by two other Mechanoid ships. The Mechanoids then broadcast a message to the Daleks on Skaro, warning them to avoid their territory. The Daleks instead began preparing for war with the Mechanoids. (TV21: Eve of War)

The Zerovians, seeking to prevent war from breaking out between the Daleks and the Mechanoids, sent their robot agent, 2K, to accomplish this. The robot manipulated the Daleks into destroying the rogue planet Skardal, which they had set on a collision course with Mechanus, then told the Mechanoids that the Daleks had saved them purposely. The Mechanoids then called off the war, although they still considered the Daleks their enemies. (TV21: Impasse)

Some Mechanoids were sent by humans to prepare Mechanus for colonization, but Earth got caught up in an interplanetary war and the colonists never arrived. Steven Taylor crashlanded on Mechanus and was taken prisoner by the Mechanoids. The Daleks followed the Doctor's TARDIS to Mechanus and battled the Mechanoids. (DW: The Chase)

The Daleks later fought Mechanoids on Hesperus. (EDA: War of the Daleks)

On Lethe, Davros combined organic tissue with Mechanoid shells to create Juggernauts. (BFA: The Juggernauts)

The Mechanoids last stronghold was on Magella. The Daleks attacked and killed all the Mechanoids, making them extinct. (DWF: Birth of a Legend)

Mechanoids do not seem to be inherantly evil nor good, as they only turn violent when their home planet appears to be threatened. Otherwise, they seem to be peaceful and open to visitors; when the Doctor, along with his companions, encountered a Mechanoid, it had them follow it back to the city, and detained them to inquire about them. However, the astronaut Steven Taylor was likely kept as a prisoner because of the methods that he came to the planet - via crash landing - which possibly caused the mechanoids to consider and treat him as a terrorist agent attempting to destroy the city.

During the 41st century, Mechanoid shells were recovered from worlds ravaged by the Daleks and brought to Station 7. At some point, a Dalek force attacked the station while searching for "the Abomination", and the Mechanoid shells were destroyed along with the station. (GN: The Ony Good Dalek)

Behind the scenes

 * No explanation has been given reconciling the conflicting accounts of the Mechanoids being human servants on television and being an alien race in the comics. One theory is that when the Mechanoids were forgotten on Mechanus by the humans, they became their own race, forgetting their origins and creators. Another is that the Mechanoids lied to Steven about their origins.
 * Designer Raymond Cusick contended that the Mechanoids were originally meant to be called the "Mechons". However, the name had to be altered due to the similarity with Dan Dare's main enemy, The Mekon. (DCOM: "Day of Armageddon") Nonetheless, the Daleks do call the Mechanoids "Mechons" at one point in The Chase.
 * The Mekon later influenced the visual design of Davros. (BBC DVD: Genesis of the Daleks)