I tested in single player so its not the mod. It seems the mob spawns but doesnt render in fully or spawn in fully, all you see is his heads
I have an IDEA, Can the Server sell the Boss Spawn eggs for MyM? and make them only allowed to be played in the Twilight
Got already reported to cauldron: Twilight Forest Hydra · Issue #79 · MinecraftPortCentral/Cauldron-Issues · GitHub
Issue got fixed: Twilight Forest Hydra · Issue #79 · MinecraftPortCentral/Cauldron-Issues · GitHub Updateing cauldron now.