not sure about it, but in orgrimmar there are invisble trolls (stealthed, 70 Elite) on the roofs of the auctionhouse and so on, i think they are there to keep alliance players from ganking lowbies in the cities.
might be something similar here?
Maybe it was or should be a mob for a quest.
Remember Behemothon from Shadow Moon Valley?
He just yells through the whole area, but it's no real mob that exists somewhere.
It's just a shortliving entity, spawned to yell a simple text and despawn(?) shortly after.
if you are lagging you might not load the name of the attacker before it attacks you and it stands as unknown in the combat log. same is with the nubish dmg meter called Damage Meters and if you are in a 5man f.e and get disconnected and when you log back in while the others are fighting all dmg done before you have loaded your party is counted as dmg done by someone named Unknown.
And another example, if you got really high latency while logging into wow inside a city f.e stormwind or orgrimmar at AH/bank you will see lots of ppl named Unknown before the real names appear.
The Invisible Stalker is probably just the entity used to trigger that portal(btw, its the SW portal, not the unused one). I remember AB use to have visible rats on top of the Blacksmith(one of the vents on top), all labeled for one of the nodes.