The pony we want to use to drive has a cancer on the underside of her tail. A krupper would sit right at the spot. The vet says he’s heard of animals driving without the krupper but I can’t find any information on it. Anyone have or heard of experience with this?
Years ago my friend used to drive her Shetland without a crupper because he had melanomas under his tail. She just sort of tied if off to the side, IIRC. I don’t think it caused any issues. Best of luck with your pony!
There is an arrangement called a spyder which goes on top of the hips to support the breeching, which has no crupper. The back strap ends in a ring which has 4 straps hanging down to hold up the breeching. I believe the spyder is a draft horse piece of harness, so you might have to get it made up special. Especially in a pony size. The person I knew using it, was happy with how it worked on her horse. Maybe a search would turn up some photos to look at. I can’t link on this tablet.
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Found this about the spyder breeching setup:
http://zinkellongearsandpack.glhenterprise.com/britchin_chest.html
Interesting site and photos. It looks like everything they do is related to riding or pack animals, not driving equipment. But the spyder idea is the same, ring on hip/spine area, with the 4 straps coming down to the breeching. 4 strap model seems by a "double spyder"in his words.
If you got one made up, I would get the maker to include a short backstrap with a buckle/billet end for the ring end, so it is the correct short back length needed for the spyder. Not wanting an overly long backstrap as needed for a crupper. Have maker put in enough length so you can lengthen or shorten strap as needed. We tend to go with longer backstraps because when horse lowers head on hills, lifts his back in Dressage, he will be making a “properly adjusted by carriage standards” backstrap way too short. Pulls their crupper too tight. Our horses really use their backs a lot, take up that slack. We don’t want them going with a wedgie!! Ha ha They would NOT be happy or go well.
I have no idea what a harness maker would charge, but believe spyder would not be as expensive as shown on the site. Be sure they stitch leather, not use rivets. Site folks are building for full size equines, so using more leather. You already have the breeching, which should be soft, double layered leather. Packing folks are using really good quality leather for strength and durability. Such leather is pretty expensive. Making up the harness spyder itself to match your harness, buckles fitting straps, won’t require the highest grades of expensive leather with your load weight so much less in pony sizes. Making the spyder should be quick and easy. Pony is not going up or down mountains to need expensive, padded strap goods like pack animals or mules do, spyders to hold riding or pack saddle loads in place in those often extreme conditions. Can’t be rubbing a hole in them, lots of miles still to go and get back.
Considering the two uses of the crupper: 1. To hold/maintain the breeching strap equal on both sides, and 2. To prevent the saddle from riding up over the withers in the case of a severe and sudden stop. - you might be able to get away with not using one depending upon the vehicle you plan to use.
A lightweight cart with wrapped tie-down straps won’t need breeching, and is probably fine in not having enough mass to push the saddle out of place at any sudden stops. A four wheel light pony carriage with a hand brake won’t need breaching or a crupper either.
The back strap is easily unbuckled and removed from the harness. All you will be left with is the saddle and breast collar.
So I’d suggest you select your carriage carefully, and accordingly for your pony. 
The necessary harness adjustment will then fall right into place.
Unless you are driving on a rough trail / up down mountains …or racing around turns …you should be fine driving without a crupper …
or
perhaps wrap it ( piece of wrap or vet wrap ) making it wider/ softer … under the tail area.
The wrapping trick worked fine for my horse when he had a sore on the underneath part of his tail.
Good Luck ~ driving without a crupper should not cause any problem.
The crupper is there to help stabilize the harness, but is not an absolute necessity. A lot of working draft style harnesses will lack a crupper and have the configuration mentioned above with the ring on the top of the hip with the hip straps for the breeching coming down either side plus the turnback/back strap. You can add a crupper onto one of these should you have the tumor removed or use the harness on another horse. Granted this is usually seen on a neckcollar harness and has two straps from the hames to the top of the spider for lateral stability.
You should be able to just replace the spider and hip straps.
I hope you find a good solution for your pony.
