Lemony Gem

Lemony Gem, or Lemony Gems, is a female unicorn pony with a yellow coat, an aqua mane and tail with a streak of darker aqua, cerulean eyes, and a cutie mark of three brilliant-cut diamonds. She is not named in the show, but she is named "Lemony Gem" sometimes with a trademark symbol and "Lemony Gems" in different merchandise and other media.

Depiction in the series
Lemony Gem makes several appearances throughout the series as a background pony, usually wearing a pink saddle with a frilly skirt that obscures her cutie mark.

Her first appearance is in The Ticket Master, outside the Golden Oak Library when Pinkie Pie reveals that Twilight Sparkle has tickets to the Grand Galloping Gala.

In Call of the Cutie, Lemony Gem attends Diamond Tiara's cute-ceañera; she eats one of Apple Bloom's burnt cupcakes before immediately spitting it out. She is seen interacting with "Dinky Doo" later in the episode. She makes brief appearances in Green Isn't Your Color at Fluttershy's fashion shows.

Lemony Gem makes several appearances in The Best Night Ever, attending the Grand Galloping Gala. In one shot during At the Gala, she appears as an Earth pony. Another shot later in the episode shows her cutie mark for the first time. In Sweet and Elite, Lemony Gem appears briefly during Becoming Popular (The Pony Everypony Should Know) in a checkerboard shot with numerous other ponies.

Lemony Gem appears in Princess Twilight Sparkle - Part 1 in the background alongside "Serena". In Canterlot Boutique, she appears very briefly during Rules of Rarity.

Trivia

 * She is the only pony that officially has "Lemony" somewhere in her name. She is also the only pony that officially "Gem" somewhere in her name.
 * She is one of four ponies whose first appearance was The Ticket Master that are seen wearing a saddle, the others being Prim Posy, Royal Ribbon, and Serena.
 * She is one of several recolors of Amethyst Star.