Trod

The Trods were a race of cone-shaped robots that ran on static electricity, led by Super Trods.

History
In 2066, the Trods were constructed on Trodos by MacTaggart to give himself power over the other colonists, secretly using them to enslave everyone. When the First Doctor, John and Gillian arrived on the planet, they were captured by the Trods, but escaped and deactivated them. (TVC: The Trodos Tyranny)

Some time later, the Trods were reactivated by another space traveller who took control of them and used them to capture the Doctor, John and Gillian. After their new Master died, the Doctor commanded the Trods to enter a furnace room, destroying them. (TVC: Return of the Trods)

Dr. Who and Jason failed to prevent the Trods from destroying Arcalis. (NA: Head Games)

The Trods constructed their own space-time machine and pursued the Second Doctor, John and Gillian to prehistoric Earth, where the Doctor used mammoths to destroy their power supply. (TVC: Pursued by the Trods)

Centuries after the first encounter, the Doctor returned to Trodos to make peace with the Trods. The Daleks found out about this meeting, massacred the Trods, and waited to ambush the Doctor when he arrived. The Doctor, John and Gillian found the surviving Trods and joined forces with them to battle the Daleks. This allowed the Doctor and his grandchildren to escape in the TARDIS, but it seems likely the remaining Trods were exterminated. (TVC: The Trodos Ambush)

Hollow models of the Trods were once on display in a museum. They were marked as "Space warmongers for generations", implying they had a wider history outside of fighting the Doctor. (TVC: The Time Museum)

Behind the scenes

 * The Trods were created for the TV Comic strip as surrogate Daleks, since the rights to the Doctor's arch-enemies had been sold elsewhere at the time – namely to City Publications, who produced TV Century 21 in which the comic strip The Daleks was appearing. Oddly, the 1968 TV Comic Annual featured the Doctor in one story with the Daleks and in another with the Trods.(TVC: Attack of the Daleks, Pursued by the Trods)
 * Their appearance as empty shells in The Time Museum is an obvious parody of a Dalek shell appearance in The Space Museum and the plot of Pursued by the Trods is eerily similar to that of The Chase.