Btukfyl

One of the solutions I think is making the muscle goes inside the bone, either by some mollusk creature using the skeletal death body or something either mechanical or organic make the artificial muscle develop inside the marrow, for example, something like exoskeleton creature.

There also a puppet string one, but that seems boring and make it seems not really a skeleton monster, so I want to know: Are there other realistic ideas or solutions? Or would this solution do or is there a problem with it?

It's an extremely rare species of caterpillar, the Beige Frontrunner.

When you look at the part that resemble human pelvis, you notice something entirely unprecedented across the entire kingdom of Animalia -- a hole! Animals don't just make holes. They're hard to make, they don't provide any benefit, and they become a liability when the creature overlooks a tree branch at just the wrong height while they're running away. Closer inspection reveals the truth. It's a pair of chelicerae! Knowing this, we can finally figure out how the entire creature's body plan.

So... since the pelvis is actually a pair of chelicerae, then what looks like the coccyx must be the creature's head. What looks like the head must actually be the tail, and the rest must be limbs! The back pair and the front pair are extremely enlarged, while the rest are shorter and serve purely as mimicry. Some of them close at the front to make the appearance of a rib cage, while some are much shorter without much mobility.

Heck why, I hear you ask?
In the long past, the creature had two pairs of enlarged limbs that served primarily as defense. The creature's unusual posture - bipedal using the front pair of limbs - has evolved from how the creature used to escape from predators. You know, 20 pairs of limbs are hard to coordinate while you're trying to hit your top speed, and lifting on your hind legs requires them to temporarily move faster than your center of mass and then slow down. The Frontrunner's ancestors would instead just push against the ground and become upright in the other way, sacrificing the temporary boost of vantage point for a quicker departure from the impending doom.

The hind pair of limbs grew longer and stronger to make the transition to upright even faster, while the front pair grew longer and stronger to provide speed during the upright phase. Eventually the creature learned a new purpose for the hind legs - as manipulators, to grab fruit from bushes and trees. This, along metabolical benefits, pushed the creature to grow longer and thinner. In the mean time, the rest of its legs just ... stayed the same. some grew slightly longer to serve as secondary manipulators - function now lost - while some shrunk to just decorative stubs. The creature did have to grow thinner as it grew taller because otherwise it wouldn't be able to support its own weight!

Then come humans. A couple thousand years ago, one group of monkeys endemic to the Red Sea region has decided it would be a smart idea to start collecting their dead to one place to reduce the risk of being detected by predators - and Beige Frontrunners have quickly discovered a new survival strategy. Because they already resembled the skeletons of those monkeys, the Frontrunners that chose those boneyards as their resting spots found themselves mostly ignored by their usual predators. When the predators began to figure things out, Frontrunners had to improve their mimicry. It was not too long before those ridiculous bulges on their tails began to evolve, as well as the coloration and unusual head structure. The eyes finished their migration to the back of their heads. Their main limbs' tarsal claws grew longer for mimicry, and they even developed local coloration that resembles the fingers of a human hand, and a stripe that mimics the gap between the radius and ulna...

As to why Frontrunners cease to function when their tail decoration that resembles the human skull gets removed ... I would also lose my mind if somebody decided to just chop off my reproductory organs!

To some degree, this is a question of what you mean by "skeleton".

Exoskeletons are well established, though really large exoskeletal creatures on land might have issues with lack of support during molting -- you might have to have them grow their exoskeletons in a different way. However, the existing way arthropods work allowed sizes up to Arthropleura (up to 2.3 m/7.5 ft long and 50 cm/20 in wide) in Carboniferous conditions. (Of course, oxygen levels were higher then, but you could just give these creatures lungs -- exoskeletons don't require tracheal breathing; spiders have book lungs, and the coconut crab, the largest land arthropod, uses a bronchiostegal lung.)

Muscles inside something like a marrow cavity probably wouldn't have enough mass, unless it was just a thin outer bone layer outside a large cavity, which more or less comes back around to an exoskeleton.

If you want a creature that looks more like a vertebrate skeleton (bones with gaps between them, not a completely plated exterior), you could have a creature with largely transparent flesh and external bony armor over only certain areas of the body (eg vital organs). At first glance, with the organs largely hidden by bone and mostly-transparent gaps between, this could look very much like a "walking skeleton".

Your question boils down to "can you convert an endoskeleton to an exoskeleton?" Tto which the answer is basically "no", because each has evolved to fit its specific task and the nature of the joints in each is quite specialised.

For want of a better example, take a look at this. Its a japanese spider crab, which I think can grow to be one of the largest exoskeletonised animals in the world.

See how all the joints are enclosed? What you can't see here is how the limbs have very thin strong walls to allow lots of space inside for muscles. In both these cases, the limbs are the polar opposite of endoskeletal limbs.

You'd need to do some considerable re-engineering, and the end result would still basically be a slightly rubbish compromise. If you had the technology (or magic) to turn a human skeleton into an exoskeleton, you'd still be better off making something more like a zombie (or mummy) or a giant crab instead. They're both still pretty terrifying, and they're mechanically much more sound so they could be tougher, stronger and faster.

Your theoretical hermit-crab-like thing that burrows into the joints of a corpse is no better off... it just wouldn't be able to exert enough force on those inappropriate joints to be strong, or fast, or tough. You could smash it apart with a big stick, I bet. It would also have to have quite a complex proprioceptive system to keep a human body upright and walking, which is a very complex operation. Oh, and there are a lot of joints; presumably it is some kind of cooperative, communal organism too. Humans have nerves to communicate with distant parts of their bodies, how would a horde of snails do it? Not very fast, I suspect.

So maybe if you wanted a fragile creature that could crawl along the floor very slowly at you, and that you could kick the head off with no effort, then you've got the right idea.