The creature was originally a human, altered by an external agent that basically remade its body (carbon based). The creature is roughly 2 meters and relatively light. It is humanoid and boneless, with muscles much like an octopus, having similar compressibility skills, but also has a series of internal structures that can soften and harden at will similarly to a sea cucumber, which are more abundant where the skull and sternum would be. Could such a thing work? If not, what changes could be made for it to be plausible? The forelimbs are much alike those of a pretosaur, with 2 "fingers" directed for sustaining the membrane instead of 1
Can a “boneless” flying creature exist?
Sure, use cartilage instead of bone. It's nowhere near as strong, but if you want to make a small batlike flyer, focusing on dropping the weight as much as possible, it's definetly doable.
The creature was originally a human, altered by an external agent that basically remade its body (carbon based). The creature is roughly 2 meters and relatively light. It is humanoid and boneless, with muscles much like an octopus, having similar compressibility skills, but also has a series of internal structures that can soften and harden at will similarly to a sea cucumber, which are more abundant where the skull and sternum would be. Could such a thing work?
Okay, so basically, not a human then. I hesitate to say no, that you cannot have a 2-meter boneless flyer, but definitely not in the way you described here. Octopus muscles are good for manipulating limbs because of a few unique structures. Take a look at the cross section of an octupus muscle - fascinating, yet unsuited for our purposes. They aren't built for power-flapping the way a bird's muscles are, and you can't rebuild them without sacrificing the way muscles are built and thus ruining the structure. Not to mention that you will need structure of some kind for the wing.
The idea you have of borrowing the echinoderms unique collagen is interesting, but ultimately flawed - the reason we have sternums is because they're always hard and they give out body structure. If our ribcage flopped on us, that would be bad because out internal organs would jumble about and since we're far more complex internally than sea cucumbers, this seems like a very bad idea and would like lead to internal complications if used consistently.
As of now, no.
If not, what changes could be made for it to be plausible?
Play around with the muscles. Human muscles are built for endurance, we need them as light as possible, so we're going to sacrifice that endurance for less weight. We're also going to restrict the diet to carnivorous so we can cut down on the needed internal organ. Swap the bone structure for something of a chitin-carapace nature, reinforcing internal cartilage. Make these beings as skinny as we dare, adjust the torso to make it more aerodynamic. This will bring it closer to plausibility.
Maybe, but not with wings.
Basically, wings won’t work because of the structural reasons mentioned in other answers. There is, however, another option, especially since these creatures are the result of intelligent design: lighter than air flight.
Simply put, they have giant airtight biological balloons like giant cysts or blisters growing from their bodies. These balloons are filled with hydrogen that has been produced with electrolysis, air that has been heated with biological processes, or some combination of both.
This would allow these creatures to float through the air like hot-air balloons; any wing-flapping they they might perform would be purely propulsive in nature so that they can do more than just drifting in the wind, rather than providing them with their lift.
If you allow exoskeletons, then yes.
Insects are boneless, and most fly.
As a genetic variation on humans you will have development problems getting from birth to adulthood.. from human infant to adult exoskeleton flying person. You can combine some real methods used by critters to make a system that could work.
Babies are born and look like babies. Gradually, their skin toughens. As part of adolescence the skin forms plates which separate and overlap. These plates molt during the final growth to adulthood.
The basis mammal body plan traces back to arthropods, which can have several sets of legs and body segments. That deeply underlying genetic code can be tapped to create vestigal limbs on the back and under the baby's skin. These vestigial limbs develop into wings, supported by chitin structures.
The internal bones demineralize but remain as attachment points for muscles, which are anchored to connective tissue to the exoskeleton.