Board Thread:Theories and Speculation/@comment-44142243-20200416151624/@comment-32685487-20200418051731

Ethereal and Spirit are pretty much the same thing, and "no one has been able to find it" implies it is something that was being searched for in the past at least. Itachi must have found them. If not it means Itachi's Susanoo was special and those were just part of his Susanoo.