ambiguous_gender barn barrel black_latex black_rubber blonde_hair blonde_mane bridle butt chastity_device clothed clothing diaper ears_up farm feral fur hair harness hay hooves indigo_furry_mix_v120_hybrid indigofurrymix_(model) inside latex_socks light_hooves looking_at_viewer looking_back looking_back_at_viewer mane mostly_nude petplay ponyplay prompt quadruped roleplay rubber rubber_clothing short_tail solo solo_focus stable standing straps submissive wall_(structure) wearing_diaper white_body white_fur wood_wall yellow_tail mechanic_31 my_little_pony earth_pony equid equine horse mammal pony detailed full-length_portrait hi_res portrait prompt_metainfo shaded three-quarter_portrait
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, [i]such a device can serve as a soft chastity belt[/i], 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.

[i]During the review of the files, it was discovered that the wrong version was published. Also expect to publish an alternative version of this drawing with minor differences.[/i]

[b]Prompt:[/b] MLP feral white pony, wearing black rubber diaper, diaper cover, farm, stall, ponyplay, harness, bridle, black rubber diaper, white body, white fur, yellow mane, yellow hair, yellow tail, tail, wrapped tail, raised tail, hooves 
[b]Negative:[/b] boring_e621_fluffyrock_v4, deformityv6, easynegative, bad-hands-5, bad anatomy, low quality, signature, logo, watermark, short legs, baby, latex suit, rubber suit, boots, socks 
[b]Size:[/b] 768x640 Sampler Steps: 20 CFG Scale: 6.00 Model: indigoFurryMix_v120Hybrid Seed: -539261158 

[b]Upscale:[/b]
Size: 1536x1280 Sampler Steps: 15 CFG Scale: 6.00 Denoising Strength: 0.50 Seed: -440205964 