Love 'em or hate 'em, it's hard to deny the cultural impact of giants. We have the trolls under the bridge, the town giants of the United Kingdom, the cannibal ogres of Native American lore, and various gigantic humanoid-shaped nursery bogies from all over the world, just to name a few. It should come as no surprise, then, that like its predecessor, this title has full-fledged giants in it. The first of these is the cave giant, a massive brute standing eye-to-eye with the mighty storm giants.
The CR 9 cave giants resemble oversized versions of Neanderthals, and have the attitudes of your stereotypical B-movie "cavemen" as well: they live in caves, are dumb as a rock (Int 4, even less than the hill giant!), eat raw food they hunt in the mountains, can't even use fire, and rule their clans through brute force. Still, the idea of an evolutionary throwback for giants has potential, and their combat abilities are definitely noteworthy. While they don't get rock throwing as most true giants do, cave giants can trample opponents (something all three of the giants in this book have the ability to do, actually) or grapple opponents before utilizing them as bludgeons.
These 20-foot titans are quite metal...literally. They are giants made out of pliable iron, and live solitary lives out in the wilderness as they forge weapons to pass the time. They seem more of a quest creature than anything else, in the manner of "hey, you want this fancy weapon? Go find the crotchety reclusive giant that lives out in the dangerous wilderness and convince him to make it". Of course, that's not to say that a ferrous giant can't hold its own in combat; after all, it's a CR 17 creature for a reason. In addition to the trample and pound abilities shared by the cave giant, ferrous giants have an immunity to mind-influencing effects and metal-themed spell-like abilities to back them up in battle.
Volcano giants have both the Earth and Fire subtypes, a breath weapon, the aforementioned trample ability, and are tribal. Other than that, there's not really that much besides the Huge size that differentiates them from fire giants as far as their role goes. I could be wrong and just too tired to pay attention, though.