This horse cake was for a girl named Emily's 8th birthday. I got the inspiration for this cake from this website. I too received an invitation and thought it was a great idea for a cake.
Here's how I made it: I started off by making 2 9x13 sheet cakes, one yellow one chocolate. I then leveled them so they were both the same size. I put the cakes together on a covered cake board. I made 2 batches of buttercream icing and made these colors: 1/4 cup dark brown or black, 1/2 cup light blue for sky, 1/2 cup yellow for sky, 1 cup beige for ground, 1/2 cup copper for canyons, 1/4 cup blue for river, 1 cup red for sides, 1/4 cup green for grass, and 1 cup light brown for border.
I frosted the blue part of the sky first, then the yellow. I then frosted the ground on the bottom of the cake. I piped on the river with a #5 tip and smoothed it out. I piped on the canyons with a #12 and smoothed it out a little, still leaving uneven spots to add depth. Then I frosted the sides with the red.
The next thing I did was sponge on a little green food coloring mixed with a few drops of water on the areas where I will pipe the grass. I piped the grass on with a tip#1 (If you have a grass tip it will take less time, but I like the realism of piping each blade of grass individually).
Now for the horse. I made the horse out of Color Flow Icing in Copper and White. Once dry, I used gel coloring and added details. I made the horse without a mane and tail, and once on the cake I piped on some hair and the hooves with a tip #3. You can use whatever colors you have leftover for the birthday message.
The last thing left to do is to pipe on the top and bottom shell border with a tip #16, #18, or #21. I used the Wilton recipes for the buttercream and color flow icings.