The Devidians were a phase-shifting species, originating from Devidia II.
A group of Devidians were discovered by the crew of the USS Enterprise-D using a time portal to feed off of humans from the past. The Enterprise put a stop to this operation. (TNG episode: "Time's Arrow"; TOS novel: Star Trek: The Eugenics Wars)
Though they frequently engaged in time-travel, their activities produced minor enough changes to the timeline that they warranted little attention from organizations such as the Federation Department of Temporal Investigations, especially since interfering in their activities often risked causing more problems than it solved. The Aegis, however, being a more advanced civilization, did investigate Devidian interference more often, sometimes even directly contacting Department of Temporal Investigations agents and telling them not to get involved. (DTI novel: Watching the Clock)