The Light Fury is related to the Night Fury species in How to Train Your Dragon: The Hidden World.
The film introduces the Light Fury as a dragon closely related to Toothless, who is identified as a Night Fury. Their similar body shape, flight abilities, and plasma-blast powers establish that they belong to related species, even though their coloring and some physical details differ.
The Light Fury is not a Deadly Nadder, Monstrous Nightmare, or Gronckle. Those are other dragon species represented in the franchise, each with distinctive anatomy and abilities. Stormfly, for example, is a Deadly Nadder, while Hookfang is a Monstrous Nightmare.
The relationship between the two species is important to the story because Toothless’s bond with the Light Fury becomes a major reason for his growing independence. It also supports the film’s broader theme that dragons have natural social and reproductive lives beyond their relationships with humans.