Horrible, filthy pony sex, you say? I think I can give a few recommendations.
As ROTHY said,
Sour Candy is excellent. There’s a decent amount of setup before you get to the action, but it’s all worth it, trust me.
By the same author as Sour Candy, there’s also
May The Best Butt Win, which is probably my all-time favorite clopfic in term of pure clop value. I can’t recommend it enough.
Sweat is really good as well, and is pretty simple to boot: Just a second-person human protagonist and Rarity, alone in a sauna.
And if you get tired of second-person main characters, try
RainBlow-Job, in which Rarity instructs Rainbow Dash on how to give a proper blowjob. Yeah, seriously. It has to be seen to be believed.
Stuck Stuck is freaking amazing if you’re in the mood for Appledash.