Today, WoW.com can report that these mobs are again visible in Stormwind on some servers. We've verified that when you view nameplates on mobs (by pressing the V key) and look up around the Stormwind bank, you can see the [DND] Shaker mob, as pictured above. The Shaker mobs are also present in the trade district. When you are able to view the nameplates, you can't view the actual mob itself -- they are invisible / have no form.
We're not sure how they work. We think they appear at random intervals, as sometimes we can see them and other times we can't. We also are guessing that they might only be able to be seen on some specific servers -- we know they're on Sisters of Elune and Ner'zhul.