Loth was an identity assumed by The Doctor in 2378. The Doctor presented Loth as the "Supreme Archon" and as belonging to the R'Kaal Imperium. He assumed this disguise in order to persuade the crew of the USS Voyager to relinquish their warp core.
In reality, The Doctor's actions were the responsibility of two members of the Hierarchy, who forced The Doctor to help them steal Voyager's warp core by holding Captain Kathryn Janeway hostage. By accessing The Doctor's holographic matrix, the Hierarchy were able to change his appearance to that of Loth. The Doctor, in the guise of Loth, stated that the R'Kaal had outlawed conventional warp travel through their territory, and insisted that Voyager eject its warp core. He later told Chakotay that if Voyager did not surrender its warp core within ten hours, his armada would destroy the starship.
Later, when the Hierarchy attempted to download additional templates into The Doctor's holomatrix, his program began to destabilize and assumed the form of several random templates, including Loth. (VOY: "Renaissance Man")