Tan Ru was an advanced alienprobe of unknown origin and great power, programmed to collect and sterilize soil samples, possibly in anticipation of colonization.
In 2267, while in deep space, Tan Ru eventually encountered the EarthprobeNomad, which had suffered a serious loss of memory after a collision with a meteoroid. When the two probes met, directives from Tan Ru were transferred and merged with those left in Nomad's memory banks. This created a probe with the unintended directive to "seek out and sterilize new life," with Nomad as the body and predominant voice during their co-existence. (TOS: "The Changeling")
Jack Treviño, a writer who sold several stories to Star Trek: Deep Space Nine, once pitched a story to Star Trek: Enterprise about the crew of the NX-01 encountering Tan Ru, before it merged with Nomad. The story was well-received, and Treviño felt it might have been bought, but this never happened, due to the cancellation of Enterprise.  However, in the season one episode "Civilization" the crew did meet the Malurians, a race the merged probe would later destroy.