This is going to be a quick one as I'm actually with my family now, but--
When packaging a model to mail, I think one of the most important things is to make sure there is enough packing material to fill the box. I use packing peanuts (I bought 120 cubic feet of them at once, that's a LOT of packing peanuts) and bubble wrap over toilet paper. I wrap ears and other protruding parts with the tissue. I wrap around the dock, for instance, in a way that will provide support rather than adding to the stress on the piece. Then a good layer of bubble wrap, then into a box big enough to have at least 2-3" of room all the way around the bubble wrapped horse. More space is better, but you don't want the box to be so big that you need a fortune in packing peanuts to fill it.
I put a good 2" or more of peanuts in the box, then put in the bubble wrapped horse, then fill the box with peanuts, tucking them in around the sides of the horse. When you close the box, before taping the top pick it up and shake it gently. If there is any shifting at all, you need more peanuts. Shifting is a danger sign--it means you model will be able to work its way to the edge of the box and get broken. So use lots of packing material so there is no danger of shifting. I tend to fill the box to slightly over the edge, then gently push the peanuts down when I close the box. You don't want there to be so much pressure on the horse that it is in danger of breaking, but you don't want it to move around either!