For which animals you are looking for: a huge ball?
Important: each horse/pony/mule (yes they play also in the USA!) size needs his own ball diameter.
My research for a horse size ball cost me about three years, and my first pony ball was too small (0.75 meter).
For which size of animals a strong ball was needed? Our pony's have the size of a New Forest Pony (Section D category), Norwegian Fjord Horse and large Welsh pony's with shoulder size till 1.45 meter. Our horses are big warmbreed horses between 1.60-1.80 meter high.
Minimum size of the ball is the high of the breastbone (just in between and above the breast muscles): so, for the pony's (Section D category) I use a ball with the minimum of 1 meter diameter, for the horses a ball with a minimum of 1.20-1.30 meter diameter. For Shetlander pony's is the size of 0.75 meter OK.
This is THE REAL SIZE, AND NOT THE SIZE PRINTED ON THE BALL! Watch out, because for instance my pony ball measures 1 meter, but the official size on the ball is 1.20 meter....
And the ball should be burst resistant, meaning: after hitting a sharp object the ball will not explode and will get empty with a light puff. The horses will not get afraid for the ball.
With some balls I have experiences: they are mostly developed for human/fysiotherapy use and NOT TESTED FOR HORSES YET!
I don't trust balls which can handle for instance a pressure of 300 kg. Because two pony's of 400 kg (in fast trot) can tackle each other with a lot of power, so the pressure on the rolling ball will be more then 300 kg.... Watch out for that.
At this moment I have found a German fysio ball for the pony's, which works well for about 3 years and once there was a leak without any sound (reason: a sharp obstacle in the arena).
My beige leather ball for the big horses (watch the picture with the Friesian horses) is handmade and not made with mass production. I found this ball also in Germany, near the Czech boarder (more then 800 km away).
These are my own personal experiences, so not officialy tested in a official test laboratory!
I'm now in contact with some European fysio ball factory's and told them how some people use their human balls for equine purposes, with all risks!
In the future they maybe wants to develop a strong burst resistant ball for pony's and horses. The market is big enough for them: Belgium, Czech, Germany, Holland, New Zealand, Russia, UK, USA (espacially for the pony's an extra ball is needed). In all these countries I have already discovered pushball lovers/initiatives!!!
The international pushball network is growing every day now. At this moment we all can learn from each other, and please tell me your story about this theme. Thanks for your comment Franky (from Belgium) and hopefully I have helped you a little. Otherwise let me know!