You can use pretty much any kind of cake for this just be sure to cool it completely then freeze it. Print a pic of the horse head you want to replicate and carve the general shape of the head out of the frozen cake. For the facial details you can carve them into the cake (will look more realistic) or build them up with fondant. Cover everything in a thin layer of buttercream so your details are not messed up and cover in fondant. Then add the bridle, etc. Hope that helps
On coolest homemade cakes, there are a number of 3d horse and some instructions.