My question is I am making my daughter a horse cake for her birthday. The cake pan is a horse shaped pan. I know it will not be enough cake for everyone to eat so I thought I could put it on top of a sheet cake.
Here is the question:
Do I have to put a divider between the two or can I just place the horse shaped cake on top of the sheet cake without anything between the two? I don't want it to cave in but I really don't think it would but you never know. Also, I plan on making this cake the day before the party...How do I keep the buttercream icing and cake to stay fresh and not hard?

no, you shouldn't need one...........

You don't need any kind of divider or support between the two. If you're doing the cake the day before, it should still be nice and moist the next day. The butter cream will crust, but that's nothing unusual.

I would recommend supporting the horse cake. Why not take a few minutes extra to insure your beautiful cake will not sag! I am one of those people that does not believe in 99.9 % of all "accidents" because they could have been prevented. Usually the prevention is soooooo easy too!
Decorating the day before will be fine and will give you the extra time you need to make sure you are really happy with your creation as well.