Here’s what I wrote about it on e6ai (at the moment, this site has become my main platform for publishing): Here is a pony in an unusual diaper, which is copied from real devices used on horses.
As you might guess, such diapers retain all secretions and do not absorb anything, but in addition, such a device can serve as a soft chastity belt, especially for mares. By the way, the illustration is most likely a mare. Another advantage of such diapers is the economic component - after emptying and washing, the rubber bag can be used an unlimited number of times.
In the case of intelligent mlp ponies, such a device may well prevent masturbation in mares, especially if there is a frame (skeleton) made of harder rubber stripes inside.
Compared to traditional, large, soft absorbent diapers, the rubber version looks really unusual. However, in the near future you can expect from me a different version of this image with an ordinary diaper.