While body harnesses are not necessary for pony play, they are very useful since they can offer many points of attachment. They look great, and the added feeling of restriction provided by a harness feels great.
Harnesses come in a variety of different styles, and can vary from those that offer very little restriction to the pony to those that are greatly restrictive. I tend to prefer harnesses that are rather open but still offer a little restriction to movement.
The harness I am wearing in the photo to the right is quite comfortable to wear while encouraging me to keep my head up and maintain good posture. This harness is typically for show, and while it does cover up the genitals, I usually wear it over a latex catsuit or a spandex suit (e.g. below right).
Harnesses for a ponygirl or ponyboy can be made from a variety of materials, but the most common material is leather. The intended use of the harness should play a big role in choosing the material and style of the harness.
A cart pony's harness should be very strong and distribute the load over the entirety of the pony's body, while a show pony harness might be picked purely for aesthetic reasons.
However, I personally advocate purchasing a strong, high quality harness regardless of the pony's discipline. A strong, well-fitting harness can still be aesthetically pleasing, with several retailers selling such harnesses, most nitable of which is Fury Fantasy.
Nevertheless, if you don't like what is available "off the rack," you can always have a harness custom made. Most vendors will help you design a custom harness to suit your needs as a pony both functionally and aesthetically.
Having a harness custom made is a great option because it will fit the pony perfectly and give the pony a unique look. The drawback, of course, is custom harnesses and leatherwork will generally be more expensive, but there are still a few shops that will custom fit a harness to a pony's measurements for a reasonable cost.
One such shop is Fantasy Leather, which, for £325 will tailor a harness (lower/direct right) with custom coloration and highlights to your specifications. The cost of priority shipping is, amazingly, also included in the purchase price (approximately $400 USD). I have not as yet tried to pull a cart using the attached rings on the waistbelt, but they seem fairly sturdy.
Other hanress makers include Fury Fantasy, TrulyHeavyLeather, and Restrictionwear. All of those makers also design their harnesses for cart pulling. Additionally, there is also FetLeather and Locked for Infinity. The latter makes the wonderful JG Leathers pony harness, which was one of my all time favorite harnesses. I never did get a harness made by him (I was a little short of funds at the time since I was at University), but I was able to afford his bridle, something I use to this day).
Moreover, most local leather vendors will be happy to work with you to create a harness for your needs. The cost is quite variable and can vary from a couple hundred dollars to around $3000 depending on the vendor, the intricacy of the design and the load its required to bear (e.g. cart pulling versus a pure show harness that doesn't have to carry any load).
A pony harness is one of of the, if not the, most expensive pieces of tack a pony will buy, so I highly advise that you take the time to look through all the vendors of harness and their designs before buying. Think carefully about what you want and desire from a harness, and do seriously consider a customized design if nothing appeals to you right away. The cost of a custom harness is oftne only 10-20% more than an off the rack harneess, and, in my opinion, if often worth the marginal cost to get something you really love.
Finally, you can make your own harness if you're willing to put in the effort to play around with leatherworking (e.g. this diy harness design), and the cost is only that of the materials.