Notes:

This is another cut boss discovered in december 2017, along with the beta Moon Presence also showed in the gallery here:

It was discovered by the same people while exploring the game's files and using the chalice dungeons as a way to access them. The boss is fully functional, named "Great One Beast" in the files and can be encountered via a specific glyph (arkhv2vs). The reason why it was removed from the game is unknown, though this is not the first time From Software leaves a fully functional boss in the game's files while not using it (like Undead King in Dark Souls 1). You can view this boss along the beta Moon Presence in the following video: