You put horse and carriage on the ferry boats! Horse needs to be less than 17h tall to get thru the door to the lower deck. My horse fit by a whisker!! Usually that same Ferry can take the carrage, but vehicles may need to get across on the flat Ferry, which is open, can haul trucks, big items, lands at a different docking area.
Floors of the deck are diamond plate, so we made rubber hoof covers from old car innertubes and duct taped them on hooves. You will need 8 covers, 4 to go over, 4 to return, since the rubber seems to get worn thru each way. When we went, horses unloaded on the somewhat narrow “people” docks which are usually VERY busy and crowded with boxes and tourists. Horse has to stay calm in this as they offload on the ramp with ferry moving up and down on the waves. You probably want to arrange commercial transport of your “stuff” to the stabling and your hotel. Otherwise it is too much to handle in that busy area. You might be able to sit on the tailgate and lead horse to stabling because I hear it is not close to the docks. Then go back to get the carriage with commercial hauler. We pulled ours behind their wagon to the stabling.
Most folks going to “The Island” have riding horses, so lots less gear to manage. We have been twice, but it was quite a while ago. Things are bound to be different now.
Forgot to mention that horse ferry leaves from St Ignace, on the NORTH side of the Mackinac Bridge. Bridge crossing can take “awhile” depending on weather, rain, wind, fog, plus the Bridge is 5 miles long! It can be exciting if you are required to have to drive beside a Semi truck because of weather. Bridge is paved on the outside lane, grid surface YOU CAN SEE THRU TO THE LAKE on the inside lane!! Exciting for me to drive across!! You call to get reservations on the daily ferry way ahead of arrival. No walk-on allowed that I know of. You can unload horse and gear in the temporary parking lot, then horse rigs need to move to long-term parking a distance away from the docks. We got gear and horse aboard. They leave EARLY, so don’t miss your boat! We stood holding horses , all stood side-by-side looking out the windows. Kind of like sardines in a can, during travel to the island. Ha ha Rubber covered hooves had no trouble staying in place on the metal deck.
They do offer public stabling now, but you need to reserve stalls ahead of arrival. Not sure if they provide feed and hay or let you bring your own. This is a different stabling area than the working horses or rental horses occupy. Your horse needs to be pretty unflappable to handle the thick crowds of walkers and bicycles, other commercial horse-drawn vehicles in town and on the roads. There are trails thru the center of the island, a paved road of about 5 miles along the island shoreline. Avoiding town is probably best, it gets very tight in there. One road in town is forbidden, extremely steep, but was well-marked with signage. I believe that private vehicles are also forbidden from driving in front of the Grand Hotel. Shoes with traction are strongly recommended, along with brakes on the carriage because of steep roads. Island horses wear rubber-covered steel which seem to give good grip, but hoof might need to be shaped to fit shoe. Shoe is not very shapable, breaks the rubber cover.
Most folks travel by foot or bicycle but there are taxis and people mover surrey though I have no idea of the rates now. You can rent bicycles too. Lots of historical things to see. There is a new building with a carriage collection and I think, the horses for the Grand Hotel carriages. The article about it was a while ago, but things looked very nice. You might check out the Sunday Brunch at the Grand. It was amazing when we visited, I felt lIke i was in a movie setting! Wonderful variety of food, great service. You do need to dress nicely for the Grand, jeans were not acceptable for dining rooms back then.