A mint green Earth-pony mare with a long, slightly scraggly mane stepped into Sugarcube Corner, having been attracted by a sign posted at the door. A much younger Mr. Cake and Mrs. Cake greeted her on the other side of the counter.
"Welcome to Sugarcube Corner! How may we help you?" Mrs. Cake asked.
"I was here to inquire about that 'Help Wanted' sign, " the mare replied, walking over. "My name is Onna. This is a bakery, right?"
"Well, yes. Sugarcube Corner," Mr. Cake answered. "Do you have a resume, by any chance?"
Onna pulled out a sheet of paper, and put it on the counter. The Cakes stared at it inquisitively.
"You've worked in bakeries before? Unpaid?" Mrs. Cake asked.
"Well, not quite. I ask to be paid in food, rather than money. I'm not looking for a life of luxury," Onna answered.
"Oh, I see. Well, if that's what you wish, we can do the same. We can put you on a trial period to see if you're suitable for the job," Mrs. Cake said, smiling. "Let's just show you around the bakery, and you'll be working in no time."
Two full weeks had passed without incident. Onna was a hard worker, and her payment was easy to provide. She even rejected the few bonuses they offered her. They would have hired her permanently by now, if it weren't for one small issue: she easily ate three times the normal amount of food for somepony her size, and she never ate with them. The Cakes had noticed this, but they were too polite to point it out. However, one day, just after she left with a large plate of food to eat in the upstairs bedroom, Mrs. Cake confronted Mr. Cake about it.
"She can't possibly be eating all of this food alone," Mrs. Cake declared. "She must have a pet or something that she's not telling us about."
"Well, why shouldn't she? We have nothing against pets here," Mr. Cake replied, confused.
"I don't know. I think I'm going to talk to her about it," Mrs. Cake said. "Be back in a few minutes, dear."
She marched off, walking up the stairs to the bedroom. She could hear Onna eating behind the door, which she apparently kept unlocked at all times. Mrs. Cake creaked open the door, and she gasped.
Onna was eating her food normally, except for the rather major aspect of her mane moving on it's own and feeding a second mouth that grew out the back of Onna's head. Onna looked up in alarm at having been discovered, and her mane settled down to normal reactively.
"I... I can... explain..." Onna stammered quietly.
"What... was that?" Mrs. Cake asked.
"My second mouth. I'm not a normal pony... though I suppose that is obvious," Onna said. She hung her head, and stood up. She walked though the doorway, and Mrs. Cake could only stare at the back of her head. "I suppose I must leave, now."
"... I honestly don't know what to say," Mrs. Cake answered. "We'll need to talk with Carrot about this."
"Of course. Then I shall depart," Onna said. "I do not wish to alarm you or your customers with my nature."
The two mares walked back downstairs, and sat at a table with Mr. Cake. Onna explained that her name was merely a shortened form of her species' name: futakuchi-onna, roughly translating to Equestrian as "two-mouthed mare". They agreed that for the sake of the customers, it may indeed be best that they didn't hire Onna, though they apologized profusely and tripped over their own words to explain that it was in not meant as a discrimination. Onna, for her part, took the whole thing quite well, saying that the last pony she's been hired by had all but tossed her out the building when she'd been discovered.
After Onna had left, it took only a few hours for another Helped Wanted sign to appear, requesting aid from a pony at least as hard-working as Onna had been. That very day, a pink mare had just arrived in Ponyville after moving away from her family's rock farm, and just happened to be looking for a job and home...