What exactly is toilet paper being used for in Equestria, other than mummy costumes and pranks on houses?
How do you know it's toilet paper and not just regular tissue paper?
Also we know bathrooms exist based on the scene with Pinkie and the Last Roundup. Considering the current technological advancement status in Equestria, there not being a bathroom on the train to Dodge Junction, and the only bathroom seen in the show being an outhouse, they don't have toilets and likely wouldn't use toilet paper, especially since it would require fine motor skills and digits or telekinetic magic.
The pony anus is remarkably self-cleaning, but only if the pony in question isn't experiencing diarrhea or otherwise loose stool. It's also still possible to get shit on the buttocks or legs, and mares may want to use toilet paper after they urinate
>don't have toilets
You fokin wot, m8?
Who says pony biology is the same as horse biology? Because as far as we can see from the series, it isn't.
I don't think actual ponies can turn their heads like ponies, for instance. So it's possible some aspects of their biology (like the voice box, for instance, and the anus) are humanoid.