You can make a mold of the unfired piece and preferrably before it has completely dried out. I like the look of my sculptures before they have lost all of their moisture. Once they shrink, they can have more grit showing etc. Either way is fine, though, dry, leatherhard clay, or even fired. Sometimes I would spray my piece with a clear sealer spray just to lock in some of the moisture and it also would strengthen some of the fine detail of my sculpture.
Above is a link to my very first mold with silicone rubber.
For a relief sculpture like you are mentioning, you can do a two part plaster mold if their are no deep undercuts. You might be able to do the same for your horse bust.