Notes:

This boss cannot be encountered under normal conditions in the game, and is most likely a beta version of the Moon Presence boss players can encounter in two of the game's endings and fight in one of them (hence spoiler tag). From Software left the boss in the game's files and there is a chalice dungeon that appears to be a test room for fighting this boss. It can now be accessed via a glyph.

Zullie's stream showing the test room with the boss:

A more straightforward look at it: