Not if you separate the horse into parts, extrude the parts, fillet, push, pull, rotate edges clever extrude and subtracts and then add the parts and more filleting. Labor intensive but doable I think. I’d have to take a shower, put my pants on and go out to my office to try it.
On the other hand if he’s printing, he doesn’t necessarily have to join them in shapr. Just lay them out correctly and tell the slicer to treat them as one object. The slicer should be able to print them merged. My slicer always asks me if I want to import a multipart model as one or multiple objects. As long as they are touching it should print as one.
Many of the ideas that @McD suggested should work, and would work in well behaved models. One thing you have noticed is that Shapr3d is not the best tool to use to work on mesh objects. You may want to try with something like Blender or Meshmixer.
I do like your concept emblem! The STL of the Mustang is, for the lack of a better word, crazy. Where the legs of the horse overlap is an issue. If you try to split the body, you get an invalid mesh, possibly a non-manifold situation. And when trying to do a split in PrusaSlicer, that section became hollow. Did I say crazy?!?
I was able to work with the model a little better by reducing the resolution, cutting the number of triangles in about half (in slicer). From that, it was easy to create a silhouette of a horse, make it thick and subtract it from the medium resolution horse.
Certainly not the resolution of a cool emblem you wanted, but you can probably get what you are looking for in a mesh tool.
