In my world, one of my creatures is a 100kg, 2 meters long flying alien. It has a durable yet relatively light endoskeleton which allows it to have proper anchoring for its flight muscles and its overall appearance seems something between a bat and an azhdarchid pterosaur. However, I'd like it to have a "backup plan", in which, should it undergo wounds that hindered it's mobility to the point it could no longer hunt, it'd undergo a "salvation process", essentially eating itself, following by a "second" creature bursting from its insides.
The "second" creature, very reminiscent in overall shape and abilities to an octopus, is in fact a survival strategy. These creatures begin their lives by leaving their chicken-sized eggs as Cephalod-like creatures, which then begin their efforts for survival. After reaching maturity and accumulating enough biomass (resulting in them becoming slightly larger than a giant pacific octopus) , these creatures will essentially enter a "pupa" stage, in a process which will result into the body "shrinking" to a smaller size and creating a capsule around it, with this emergency body housing most vital organs and structures, such as the brain, lungs and digestive track. The "emergency body will have its excretion openings linked to the main body formed around it, as well as having linked blood vessels and nervous system (at this stage, the emergency body essentially acts as a second layer of protection to the main organs and lies in a dormant stage, with the main body truly housing only this emergency body, the necessary musculature, the skeleton and the reproductive structures, all naturally covered by a second layer of skin). Upon major damage to the main body, the "emergency body" will salvage as many nutrients as possible from the main body, sever these connections and burst out of it, starting over its cycle of growth and metamorphosis into its "adult" form. As an adult, the emergency body occupies most of the abdomen, where on earth would be the organs, and part of the skull. What I'm willing to achieve partially reminds me of of the Krang, a fictional species of soft aliens which build themselves robotic bodies for protection and were a good example of what I'd like my creature to be.
Summing up, could a creature with such a life cycle and defense mechanism involving housing a body inside another possibly exist in any way?