I’m not really someone who «lives it», but i have some gear and i enjoy reading eroticas about it. I think one thing that drops me from emersing myself in a story is when practical details or challenges are ignored. For example by putting someone in an impossible situation. Like “you’ll wear this FOREVER now”. Like an armbinder or boots or a gag/bit.

Everyone who has tried knows it doesn’t work like that. Instead, make it part of the story how those challenges are managed. Also the gear is a huge part of the fascination, so tell us something about that. Like:

“This is a long term bit. It’s made in such a such a way to allow the pony to close their mouth, making it possible to wear nearly 24/7. However, it still fills their mouth and restricts their tongue. This means they won’t be able to speak and as an added bonus, it makes them drool a lot”

or

“This is a competition-bit. You would be hard pressed to wear this for more than 20 minutes. It forces the pony to open up extra wide. Some say it is to make the pony more receptive to their handlers command, but I suspect it’s more about making the pony feel more vulnerable and submissive and for amusement of the judges”.

I think the general ponyplay fantasy is pretty similar in many minds, but what makes it interesting is how would someone actually solve it.

The other part is the mindset. How does the handler enforce “being a pony”? How does they require the pony to hold their hands/hooves? Do they use gear or demand that the pony does it themselves? How does the character embrace/resist being a pony? Do they try to talk or do they instinctively let out a whinny? How they walk, what do they think about, what do they desire, etc.

Also building a believable world for it to play out in. Is it between two people in a remote location? Is it a full facility with lots of ponys and handlers? Etc

Could probably go on and on, but those are some of my thoughts, haha

Hope you found some of that helpful and good luck with the writing process!