You would need to make a mold and then pour the resin into the mold and wait for it to dry. I guess you could find a spring at the hardware store. Might be a good idea to find a cheap bobblehead and take it apart to see how it was assembled.
I don't know about bobblehead horses, but Saratoga this meet just gave out limited edition bobblehead great jockeys. We got Angel Cordero, opening day no choice, if have owner/press /whatever, your choice. Another Angel.
I've never collected them, but I've always liked them. I wonder whether you've heard of Drew Soicher. He's the 9NEWS sports anchor here in Denver. He always uses bobble-head dolls in his broadcasts. He apparently has hundreds of them. Have fun with y...