This appears to be a very niche or private release—likely a based on the naming pattern.
This document provides a comprehensive overview of Equellum Fabula: Carmen Cygni Equellum Fabula- Carmen Cygni -v0.4.1.0- -Gaiki...
Equellum/Fabula: Carmen Cygni is an adult RPG developed by Rozen focusing on a story-driven fantasy plot involving a "futanari horsegirl" navigating the town of Umikaze. The 0.4.x.x cycle (Centaur Plains Update) introduced new chapters, the Centaur Plains area, new party members, and a rework of the Maruzone Shrine. Detailed development updates and content descriptions can be found on Gaikiken's Fanbox page . visual novel, interactive fiction, RPG Maker project, or
While primarily a Visual Novel, Carmen Cygni integrates elements of adventure and light RPG mechanics that serve the story rather than distract from it. In version 0.4.1.0, the exploration segments have been refined. While primarily a Visual Novel, Carmen Cygni integrates
Putting it all together, "Equellum Fabula- Carmen Cygni -v0.4.1.0- -Gaiki" could be the title of a creative work (story, poem, game, etc.) that combines elements of fable and song, possibly with thematic elements related to swans. The version number suggests that if it's a digital project, it's still in development.