Is there a mod to stop my own horse breaking my hidden state while sneaking?

You mean in that you are detected when the horse is in the line of sight? The enemies do not see you because of this.

Now, if you mean having the horse run and and do battle while you are trying to get some sneak kills in, that's a different story.

Here is a list of Horse related mods

http://www.gamesas.com/topic/1459682-relz-wrye-bash-thread-97/

I saw something that the Better Horses mod was on hiatus, I think this was the one to take up that project

http://skyrim.nexusmods.com/mods/14950/?