The character of Princess Celestia was originally supposed to be Queen Celestia, but Hasbro executives decided against the title of Queen since Disney had started the trend of animated Queens always being villains(excluding Elsa from Frozen).
Applejack is the only main character to remain mostly the same to the character she was based on. The other five main pony characters are all amalgamations of several characters from past generations of the show
Producer Lauren Faust developed this series with the goal of creating a more sophisticated version of the My Little Pony franchise with more realistic and proactive characters than the usual sexist stereotypes she saw in earlier incarnations. For instance, the insistence on the part of Hasbro executives that clothes and fashion be a part of the characters' interest was addressed by making Rarity obsessed with fashion in the context of being a professional artist and a small business owner with clothes as her preferred medium.
Lauren Faust originally wanted to call the villain Nightmare Moon; Discord. But executives from Hasbro decided the name would be too confusing for their target demographic to comprehend. However when pitching for the second season premiere, Hasbro execs had no problem with the name when Executive Producer Lauren Faust mentioned the character was akin to Q from Star Trek: The Next Generation (1987) The comparison led to show producers actively seeking out John de Lancie to voice the character.
Andrea Libman does Fluttershy's singing voice, but not Pinkie Pie's. She stated the reason is because Pinkie Pie's high pitched voice tends to be a strain and that singing with it really hurts her throat.
One regular background character pony with a hourglass cutie mark was noted by fans to have a distinctive resemblance to David Tennant's version of The Doctor of the British TV series, Doctor Who (2005). As such, he has taken on the fan nickname, Doctor Whooves, and is the subject of numerous fan creations, although his official name in the series is Time Turner. However, Hasbro has released an official figure of the character under the name Dr. Whooves with a collar and tie to accentuate the resemblance to the 10th Doctor. In addition, he appears in at least one variant cover of the official My Little Pony comic book with numerous references to the British series.
Because of the shows overwhelming popularity, which generates a high volume of fan-related content the show creators have to do their research to make sure any one of the several pre-existing characters they wish to feature in an episode doesn't use a name that in any way sounds similar to one created by the fan base.
Rainbow Dash is the only main character to feature a "cutie mark" that is a single symbol. Rarity, Applejack, Fluttershy, and Pinkie Pie all have "cutie marks" that are displayed in a cluster of three. Twilight Sparkle has a huge sparkle surrounded by five little sparkles.
Pinkie Pies original design was a white Pegasus named and based on Gen 1 pony surprise but due to Hasbro no longer having the trademark changed her to g3 pinkie pie' colors and name later it wad decided to move her wings over to Posey based fluttershy. Some early show bible images used old concept art for suprise in pinkie pie's colors to represent her.
While the show is labled for younger female audiences many of the jokes and references in the show have adult themes to them. Some include references to alcohol, movies for adults (like The Big Lebowski), and a few references to older bands and music (like Diamond Dogs).
The fan favorite pony, a light grey pegasus mare, with blonde hair and a bunch of bubbles as her cutie mark, has many names. The fan name is Derpy, or Derpy Hooves, but that name was used in the show once, but some people found that name offensive (but derp/derpy means clumsy), so her show name is Ditzy Doo. But, on merchandise, her names is either a picture of a muffin, her face, or "Bubbles"