These are your Dust Devils, your Ice Elementals, your various Lava Elementals. There's not really a lot to say about them. They're composed of more than one of the four primary elemental types (fire and earth, or air and water, or air and earth or what have you) and they tend to be created by the conflict between each elemental lord. Ragnaros, for instance, is responsible for most of the lava elemental types as he attempts to expand his holdings on Azeroth by melting away Therazane's earth for his own purposes in his Molten Core. However, Lord Ahune shows us that Ragnaros is not the only elemental lord capable of creating or controlling such entities, as Neptulon clearly has the allegiance of the Frost Lord and his commingled water and air form. Some elementals are even said to be composed of more than two elements, with the expected divided loyalties.
This is a catch-all category for a wide variety of entities that seem connected to or bound by various elements or elemental lords but that are not elementals per se. It's clear that the Hydra is considered sacred by the Twilight's Hammer. (If you run Blackfathom Deeps, you'll get an interesting bit of lore about Aku'mai, a hydra and one of the Old God's favored pets.) The Hydra's connection to the element of water is such that two elementals take the form of hydras, or possibly are hydras promoted in service to various elemental councils. We've already mentioned Flamewakers like Ambassador Flamelash who seem to perform a similar role for the element of fire, as do core hounds such as The Beast.
This basically means "I have no idea what it is." This is a category for beings like Murmur. Murmur is apparently an elemental of sound, an entity as old as the creation of the universe who doesn't really fit into more comprehensible elemental systems like those of Azeroth and Draenor. It's worth noting that Murmur's very presence is so disruptive that the vast majority of the Shadow Council forces in Auchindoun spend a great deal of time and effort utterly failing to subdue him. Murmur is just one of the many disturbing entities mentioned in the Codex of Blood, implying that there could be other elemental forces like him, essential lords of things we have yet to grasp even could have an elemental nature. (Also, if you're a Robert E. Howard fan, Murmur is very reminiscent of a Kull story, "The Screaming Skull of Silence.")
When we first saw revenants in Warcraft III, they were pretty definitely elemental-aligned undead. The Guardian, probably the most famous revenant, sat in Frostmourne Cavern and did his level best to prevent Arthas from picking up that infamous letter opener of his. As we've progressed through Wrath of the Lich King, we've seen revenants in Zul'Drak, serving Loken in the Halls of Lighting, in the very forge of Ulduar itself. It's now a lot less clear if we're to view revenants as undead at all. They seem to be purely elemental in Wrath, often similar to but distinct from pure or commingled elementals, with crude armor created from the element or elements they personify. It doesn't help that the Scourge is invading right next to the places we find revenants in Zul'Drak, or that Frostmourne Cavern still seethes with them.
It seems pretty clear that even potentially unaligned revenants like Ichoron are fairly sinister in their goals and motives (which might explain why the Kirin Tor has her locked up). This might in part be due to the fact that the revenants seem far more loyal to the Old Gods and their ancient war with the Titans than the other elementals are. You don't see many revenants to date in the armies of the various elemental lords, in fact, although with Cataclysm on the horizon, we may start to see them pop up.
That at least covers the basic divisions between elemental types. And that brings us to an end of our six-part exploration of the elementals, just in time for Cataclysm to shake the very foundations of the world and split apart the titan's division between the Elemental Plane and Azeroth once and for all. Next week, I talk about something else entirely!