If you plan on making this out of EVA foam, I would look for a (hollow) 3D tail with a similar shape such as a velociraptor tail, divide it into segments with a ball joint attached to each of them interlocking through letterbox opening in the next one (split each segment in two, so you can reconnect them with magnets while enclosing the ball joint of the latter), and fuse the first segment with the pelvis/cod of a mannequin/3D model.Plastic-coat the pelvis/cod with first segment for strength.You can then easily wear it underneath old trousers and cut a hole in the back for the tail.
Just my thoughts on this concept.Good luck with your project!