“You asked what it was like living with Wisp. It’s messy,” Spike clarified as he chewed on the diamond, licking at flakes of the white stone that crumbled between his teeth. “Wisp isn’t toilet trained yet,” He said with a shudder.
“And that means?” Dash trailed as she returned a blank expression from across the table.
“What goes in one end has to come out the other.” Applejack answered dryly.
“That’s a problem why?” Rainbow Dash didn’t seem to comprehend as she sat back down again. “Wouldn’t it just go through the cloud deck… oh…ground pony.”
To put it mildly, Applejack was stunned. She could only stare at her feathered friend. “You mean to say that you pegasi just let yer foals do it on the floor and it falls out of the cloud!”
Oh, the rotten luck that earth ponies had! Unicorns had their fancy magic to pick up things, but to hear that pegasi just did their business and it would take care of itself was almost as infuriating.
Earth ponies had to change dirty diapers with their mouths!
Their mouths for Celestia sake!
Applejack remembered a certain younger sibling and the diapers she helped her mother change. Nopony would ever forget smelling something like that, especially up close.
“Hey, we have indoor plumbing! Rainbow recycling.” Rainbow Dash protested. “We don’t poop on other ponies! It’s perfectly okay to walk under a cloud house or a pegasus town. Blue ice is a racist myth!”
http://www.fimfiction.net/story/28718/10/the-foal-in-the-basket/twilight