I'm assuming you mean something to sit on -- like a lift? As mentioned above, there is a little step, but it still involves climbing. I did some research and found that while it does appear possible to make a buggy or carriage wheelchair accessible (with a lift or ramp), I've found no mention that either the horse-drawn carriage or wagon rides at WDW are equipped like that. There have been a few questions posted here over the years but no definitive response indicating accessibility features.
If there is something new, and/or someone does have info about accessibility for the carriages, hopefully they will post.