For the seahorse, my goal is to make a sea horse thats tail straightens as it swims. An idea I have is to cut the cardboard tail into different segments and attach a string, slinky, or a wooden snake for movement of the seahorse. 

For the hedge hog, my goal is for the spikes to work. I could do this using the same texturing as a rubberball with spikes on it. It also could be done with springs as well. 

For the tail of the sea horse I would make it similar to the last image.