The earth pony was starting to get uncomfortable in this unfamiliar world, and body. He wanted to go back to his own world, he was more comfortable there, and he wanted to do it before he'd need to piss as he wasn't about ready to figure out how ponies do that. Unfortunately for him, his body decided to go the other road instead, he had to shit. He could have asked his partner for help with that, but he didn't want to, he was too shy, and didn't think it was that bad, he could wait. He kept following him through the town as he talked about it.
After what felt like hours, but was closer to just one, the pressure building up inside his bowels was starting to really hurt, he didn't think he could actually make it to his world, and even if he managed to magically get back there within a minute, he'd still have to drive home. That wouldn't work out well. Instead of being smart and just speaking up and asking his partner for help, he ducks off and runs to a nearby clearing and behind a tree in the grass, if he couldn't open his damn mouth, he could just hide from everypony instead as he figured it out on his own. He's not unfamiliar with pissing or even shitting outside, but, he's not familiar with a pony body, so he isn't sure what he's supposed to do.
He doesn't have to think that hard though as his body figures it out for him without any effort and instinct kicks in. His tail raising up and out of the way of his anus, that is already pushing out the first of the shit waiting inside him. He raises his hoof and bites his sleeve to stop himself from making any noises and potentially alert somepony to his presence, as it feels all too good to not be on the brink of explosion.
While this is taking place, his cock slowly drops itself from his sheath for a reason he doesn't understand at first. He thinks that maybe he's just a freak in this world and somehow getting off to taking a shit, but in fact he's wrong and it's not even erect, it's because he needs to piss too, which he hadn't even noticed as the need to shit was far stronger. He'd gone this far already and he didn't really care anymore, so he let himself piss too.
His bowels had managed to empty themselves before his bladder did, but both were finished pretty quickly. He sighed with relief and put his hoof down, he felt weird and gross now, but at least he wasn't in pain or still trying to figure out to take a shit as a pony, which as it turns out is not too different to that of a person, just with more standing up. He waits a minute for his cock to retract back into his sheath before he walks away as he wouldn't want that to be seen.
He rejoins his partner and is about to think of a lie for where he went when he gets told that he saw the whole thing and that he should have just said something. If it was possible to die of embarrassment, the earth pony would probably be the first pony to ever do so.
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