Last time I went to Grol’dom Farm (in Northern Barrens) I there are some mobs (Mercenary Engineer, Kor’kron Incinerator, …) that were level 90.
Today I went to that zone, and I found that they are level 60. Even the loot has changed!!!
Barrens is a scaling vanilla zone, which means it’s mobs scale up to level 60 (if your character is 60 or above).
In addition to the level <=60 content, there are a couple camps with Mists of Pandaria level ~90 mobs that were related to the patch 5.3 Battlefield Barrens / Kor’kron content. Much of that stuff has been removed since, but there’s a couple spots left where you’ll still find these level 90 dudes.
I’m not sure how/if they show up for lower level players, or how Party sync affects them or not.