Are you sure it's cake? It looks a lot like a chocolate mold to me...
As far as molding fondant/gumpaste by hand, I just did it for the first time today. You just play with it like play-doh until you get the shape you want. Since mine was standing up I did a lot of round balls and sausages to get the shapes I needed. Where two pieces connect I tried my best to blend it in by thinning the edges into the main body. Do not forget to use gumglue if you are trying to adhere two pieces together. I posted my piece below, it will be painted to look like a tiger.
I agree with Odyssey, it definitely looks like chocolate mold to me. Horses have skinny legs and don't stand up on cakes very well at all. They tend to break quite easily.
just seen on an episode of Ace of Cakes where one of the girls made a horse. she started with a wooden dowl body with a round wooden middle section and head. she then put modeling chocolate on the entire thing. it looked good, and provided some added stability. HTH
If you are wanting standing up horses, I made the deer (in my pics) by forming an armature out of that fiber-covered wire found in the Michael's artificial flowers/ oasis foam, florist's tape aisle. It comes in coil and looks like grapevine. I formed the body shape (head, neck and back) in one piece and cut two lengths 2x the length I wanted the legs to be. These I wrapped around the body at the appropriate places, crimped tightly with pliers and bent into leg shapes. Then covered with modeling chocolate and finished the details with carving tools. I was upset because they were supposed to be deer and ended up looking like horses! I hope this helps you.