There are shitloads of giants in the monster manual so I can’t see how one more will hurt. Seriously, it seems like every possible type of terrain got its own giant. Mountain giants are not hill giants, forest giants are not jungle giants. There are no garden giants, and no terminal moraine giants. No archipelago giants. I feel like glacier giants deserve to be their own thing, though they seem to have been absorbed in the rather-unimpressive frost giant. Beach giants must be a combination of desert giant and reef giants. There is no city giant and no moor giant.
Arnold K already did a very good post about very giant alien giants who swim through the earth. Go and read that first.
Cave giants can no longer walk. Naked, hairless and pale. Full-body scar marks cross the thumb-sized varicose veins that wind across their skin. They crawl, if they have the space, on deeply calloused forearms and knees, pausing often to listen and sniff the air. Their bones have turned to flexible cartilage like a sharks and will no longer support their weight when standing up. They can, however squeeze their way around underground, passing through narrow gaps that would trap any other kind of giant. This giant could come into your house though the front door. It could slowly squeeze its body up your stairs, filling the stairwell with its flesh, then send one questing hand through your bedroom door. It could squeeze its head into your room and look at you.
Cartilage deforms more than bone so a Cave Giant can, with time, work its way through spaces that in human scale would be little bigger than a letterbox mouth, about the size (relatively) of an A4 book. It could come in through your window like a burglar.
They crawl around, semi-blind and sniffing constantly with their remarkable sense of smell. They have not yet fully lost their sight as long lifespans means giants evolve (or devolve) quite slowly.
If they could stand they would be about 18 or 20 feet tall, but slender and starved. As the crawl, their eyes will be about five feet off the ground, level with yours.
They have lost none of their strength. They climb well, oozing and creeping up the rock with every point of their flexible body in contact. They lack leverage due to their flexible bones so usually choose to strangle, crush or twist apart their prey.
You may be attempting a passage just low enough to make you crouch, and see, ahead of you, a pale gigantic hand reaching towards you, clutching at the rock. Behind it an arm, a shoulder and then a gawping face filling the width of the passageway, rolling opalescent eyes under half-closed lids. A mouth like the boot of an economical car.
What to you, is a walking passage, to the giant is a dangerous squeeze. It must lie flat, with one arm extended out and the other pressed back against its side. It lets the stone scrape and compress its cartilaginous skull and distend its head to the corridors shape.
If you will not, or cannot retreat, the giant has no choice, it cannot turn around. Its only option is to crush you against the wall with its outstretched hand, or grab you and squeeze you to death, then to slither forward and scoop you up in its mouth. It must eat you, chewing well, equipment and all, to get you out of the way.
There are very few Cave Giants and they must move constantly to find food and avoid organised resistance. They call to each other by finding hidden seams of rock, biting into them, and screaming into the stone in ultra low-frequency. It’s huge body, and its wide contact with the stone let it sense low frequency waves reflected from the strata.
The rest of the time they are silent, like much cave life. Highly intelligent, they exchange much information with their strange long howls into the rock, mainly about threats, prey and changes to the environment. They are of neutral alignment, but, like everything underground, they are constantly hungry. If you could find one after it ate, you might possibly be able to negotiate, though there is nothing they want.
They are loathed perhaps less than they should be, as they hunt Fomorians. They stalk them invisibly from the dark, needing no light, they wait for long periods, days, weeks, or months, without moving. They watch from some impossibly small, door-sized crack, noting the movements of their prey. Then, when all is still, they creep out, crawling silently towards the sleeping freak. They slip rubbery fingers round its neck and choke out its life. Then the drag it away to consume. If necessary they slowly twist off limbs to get it through the gap. Cave Giants have perfected a way of twisting the limbs from a gigantic corpse so that the skin knots at the joints, preventing any flow of blood and meaning they leave no trace as they are carried away. Very rarely, the Cave Giant plans badly and the trunk of the Fomorian is too big to fit. They leave it propped up like a present, arms, legs and head twisted off with neat little fleshknots where they were. They carry the limbs off like a string of sausages.
Fomorian slaves never wake their masters while a cave giant crawls towards them. They know the giant will often release them just before it leaves, though this is to create chaos that will mask its trail while it escapes.