There are organized rides in most, if not all, states. Try googling "equine clubs in MI" or "trail rides in MI". Anything like that should get you to some links. Equine clubs have all sorts of organized things and there are many weekend or daily rides that you can find with a little help from google.
When I moved to SC from PA, that is what I did to find trails, and riding partners, as well as events.
Get yourself involved in your local competitive trail and endurance club.
If you meant something more along the line of a 'horse rally', I know of a large beach ride organized each November (I believe that's when it is) in South Carolina. I've ridden it and people bring every breed imaginable. Some people even drive their horses down the beach.