Threes are important in nightreign, and the third phase gnoster fight further illustrates this.
Basic gnoster involves two entities with a shared health pool. The third phase involves three entities with their own health bars, but the new one can heal and revive the others.
Gnoster would seem to be from the same root as gnostic; knowledge or wisdom, basically.
The new phase both differentiates and names the second entity, faurtis, and introduces the third, animus.
I'm not entirely sure what faurtis means, but I'm going with "fortis" - strong or fortified - because animus would seem to complete a "body, mind, spirit" deal with the other two. Animus usually means ones instincts or motivations, but can also mean animosity or, in old timey "sheltered rich guys guessing about things" psychology, the male aspect of a female psyche.
It's pretty easy, at least for me, to read gnoster and faurtis as drangleic creatures. Gnoster is pretty obviously a moonlight butterfly turned into a boss monster. In ds2 these were enemies that, in most places, would be non-respawning, hidden enemies that drop healing items. In the huntsmans corpse they appear as respawning enemies that cast poison cloud and drop poison moss. Faurtis could relate to tark and najka, both found in the shaded ruins but associated with freyja and, by extension, Duke Tseldora (Aldia, probably). Tark najka and freyja were attempts to apparently create lordranic demons in drangleic. Since faurtis doesn't have a humanoid rider I'm going to guess it's a similar species as tarks lower vinegeroon half and, as such, capable forming permanent symbiotic bonds with other creatures.
Animus I need to think more on but: gnoster's pilgrim butterfly?
W/r/t lore: I set out a case a long time ago that Carthus, the desert empire that took over most of the world, began in fallen Alken under the reign of Mytha, who was the, uh, head of the snake of an alliance of the brotherhood of blood, jugo pyromancers, and the grave wardens. Based on jester Thomas behavior, it seems like queen Mytha was set in opposition to queen Elana. Elena's faction seems to have gone on to become a kingdom that would, via reverse colonialism as a result of the rise of and opposition from carthus, become lothric.
Huntsmans copse, if it was the birthplace of carthus, matches the description of the forest that became a desert gnoster could have come from, and the shaded ruins are framed as desertifying due to various influences.