The Great Globin: A Maiar?

[Tinman]

According to Wiki: In letters written later in his life Tolkien suggested that the Great Goblin and other highly influential leaders among the orcs may not have been mortal orcs, but lesser Maiar who had taken orkish form, or "hröa".

I can't seem to find the particular letter referencing this, or any other detail hinting the origin of any goblins being a source other than the normal debated theories. Can anyone share any further details?