There has been materials science research on materials that can harden and soften at need. In nature, we have the example of the sea cucumber, which can go from a squishy compressible state to a rigid state through special fibers in their skin. 

Knowing this, could a humanoid predatory organism with an endoskeleton that softens at will exist in any way? If not, how could it still be agile on land while being able to fit into tight spaces like an octopus? I'm not thinking about natural evolution, just an alien bio-weapon.