I get this question at least once a week.
The answer is: A Pleasure Style Harness. This is because the line of draft on a KBike is fairly horizontal.
I have written quite a bit about line of draft. This post has the most info all in one place: Collars, Line of Draft, Balance of Draft
A pleasure style harness is one that has a soft breast collar. A farm/work style harness is a harness that has a collar and hames.
Here is a blog all about breast collars that I wrote some years ago: Breast Collar
The harness shown below is the Trail Harness that I designed:
It uses a SuperFlex Euro style breast collar. This style of breast collar is very soft and is ONLY appropriate for a horizontal line of draft. If you try to use this style of breast collar for dragging heavy weight, with a low line of draft, the breast collar will collapse - both at the top and at the bottom. This will put ALL the weight on the top of the neck, while collapsing into the wind pipe at the bottom of the neck.
When used with a horizontal line of draft the breast collar can function as it was designed to function. It is important the the collar be a bit snug and it NOT be used to pull a lot of weight. Luckily the KBike only weighs 55 pounds! So it's unlikely you will overuse this breast collar with the KBike.
I HIGHLY recommend using a harness that has a breeching. The breeching acts as the brakes with the KBike and any other style of two wheeled vehicle. NOT using a breeching is only setting yourself and your pony up for trouble the more you drive.
The breeching is the part of the harness, shown above, that goes around the pony's rear end. You will have hold back straps that either buckle or snap to the ring on the breeching and wrap around the shafts of the KBike.
Here is a link to the blog I wrote all about the breeching: Breeching
The KBike has a singletree that requires Quick Release Shackles. You can purchase these at Chimacum Tack, My own website, www.idriveponies.com, or from most sailing/boat companies online. The more they cost the better quality they are.
The ones from Chimacum Tack require some thread locker on the screw that allows you to attach them to the loops on the singletree: Quick Release Shackle Care
Here is a link to the ones I sell: Quick Release Shackle This style has a ring, similar to a key ring, that you have to pry open, then you can release the pin and slide the opening over the ring on the singletree, then you put the ring back on the end of the pin. This makes it MUCH harder for the QR shackle to come off or be removed. I have never had any issues with these shackles.
This is the most expensive option. This style should never give you any issues if you can afford it: IVC Sprenger Quick Release Shackle
A quick video that shows HOW the shackles work:
Now, what kind of traces do you need for the KBike? The Combo end trace sold by Chimacum Tack. Here is a blog that shows the two styles of traces and also discusses what piece you can add if you have the slot end traces: Traces - Combo End or Slot End?
As much as possible I try to work with you and the harness you currently own. Purchasing a KBike does not mean you'll need to purchase a new harness also!
So to sum up: A pleasure style breast collar is the appropriate style of harness to use with the KBike.
If you have a breast collar style harness right now it will work with the KBike. You don't need to get a new harness just for the bike.
If, however, you are interested in purchasing the Trail Harness, as shown above, please head on over to Chimacum Tack: The Trail Harness
If you have any questions please feel free to reach out!