The Abductors, or Reticulii were an advanced civilization who were known to have visited Earth in the 20th century, notable for performing medical procedures on unsuspecting Humans. They sought to undo the ecological damage caused by the Preservers.
When first encountered by a Federation starship, the Abductors displayed a huge power capability, completely devastating the Constitution-class training vessel USS Yorktown. Cadet Daniel Ehrlich had assumed command of Yorktown and the few survivors of the crew when Rear Admiral James T. Kirk and the USS Enterprise arrived on the scene to investigate the alien intentions. They found the Abductors had performed numerous medical procedures on Humans throughout Earth's history. The incident gave Kirk the inspiration to take a new position at Starfleet Academy, because of the poorly trained cadets who botched the encounter, as well as the threat posed by strange aliens requiring his input to the staff at Starfleet Operations. (TOS - Untold Voyages comic: "Odyssey's End")
By the 24th century, it was known these aliens, called the Reticulii, had conducted a series of genetic profiling experiments on Humans at that time. In order to safeguard their experiments, performed on medical ships in orbit of Earth, the Reticulii inflicted Missing Time Syndrome on the abducted individuals. (DS9 - Millennium novel: The Fall of Terok Nor)
Both James T. Kirk and Jean-Luc Picard were aware of Reticulans and Vulcans visiting pre-warp drive Earth when they discussed the Prime Directive with Data in 2375. In particular, the Reticulans were noted as an exceptional case, having interfered with Human development where other visitors had adhered to non-interference directives. (ST novel: Preserver)