Watched this video (see attached link) from Season of Arrivals, thank you Destiny Lore Vault, within the Prophecy Dungeon Eris confirms something interesting information about the mysterious creature on Titan. From this we know three things: 1) The "long boy" is 100% a Leviathan and not a Hive Worm. 2) The "long boy" isn't a boy at all, but in fact a girl. 3) She's been on Titan a long time.