I’m just going to speak honestly from my experiences with being the only driving person at barns full of riding horses…
To me, it doesn’t sound like your mare over reacted or acted in any way unusual or frightening. It sounds like you’re a little intimidated by the mare unless she’s absolutely perfect, and if you’re unconfident about handling her, how can you possibly give her the confidence to see something new and conquer it?
At the barn I’m at, I drive a mini (well 2 actually), a pony, and a horse. Sometimes a neighbor comes to visit with her horse and 4 wheel surrey. A whole lot bigger than my meadowbrooks & EZ entries. The majority of the horses at my barn (14 of them to be exact) are all OTTB’s that play polo. Some of them are older, but the majority of them are less than 5, and arrive to the barn within days of their final race. They are as fit and hyped up as thoroughbreds can get. And believe me, they’ve never seen anything like a horse-drawn vehicle, or even minis.
They all spazz a bit the first few minutes they see the cart (from their paddocks). By the time a few days go by, even the newest arrivals, two 3 year olds that just got there last week, are bored to tears with the driving horses. The outdoor arena, and the main driveway border the polo paddocks on several ends, and share fencelines.
I have personally never met a horse that doesn’t “get over it”, when exposed to it properly. They all freak out the first few times.
Others here have given you valuable suggestions - especially the one about walking the mare behind the cart.
Recently at a show, the warm up grass area was shared by both driving & riding horses. I’m sorry to say, one of the young TB mares being schooled, flipped out over seeing me and the mini enter the area and dumped her rider. Instead of getting angry that the dreaded mini spooked her horse, the rider came over and asked how she can get her horse used to it. I told her to lead the mare behind us while we walked. Within about 3 minutes, the mare took a big, bored sigh, and was done.
I used to pony a 3 year old behind my cart, with her owner sitting next to me, holding the lead. Sounds a little unsafe, I suppose. Probably was, but it got that young mare absolutely dead about carts !
Maybe your mare may take longer than a few minutes, but DRUGS are NOT the answer, and really make me think you are fearful of your own abilities to handle your horse.
You maintain that:
So I don’t need to get her used to new objects,
But yet, you would…
just like for her not to get riled up if this mare is worked w/ the cart again.
So isn’t that a bit of an oxymoron in a way? Clearly, not only does your mare need to get used to dealing with new objects- but you need to learn how to grow confident about exposing your mare to “scary new stuff”. You came here to ask advice on what to do about getting your mare used to it, but you seem unwilling to admit you have anything you need to work on.
Anyway, that’s my opinion based upon my own experience. Agree with it or not.