Fluttershy Toy Prototype, Image by Eskara1862
Thanks to a question from ShortyBoo, Sibsy has revealed the name of the G4 yellow pegasus in her MLP Arena introduction thread: Fluttershy!
This pony has been the subject of considerable confusion, due to her color similarities to Posey, a pony originally released by Hasbro in 1984 or '85. Prior to being hired as creative steward for "My Little Pony: Friendship is Magic", Lauren Faust posted a drawing of Posey on her deviantArt page. A very similar piece of art was incorporated into Brittany Harriman's draft "Licensing Style Guide for the new 2010 My Little Pony brand", with a variation in the flower symbol.
Enlarged Licensing Style Guide Image
These images caused many pony fans to speculate that Posey would be a character in the new series, especially in consideration that other 1980s characters Applejack and Spike the Dragon would be returning. However, Lauren Faust had said,
"Except for Applejack and Spike, Hasbro lost all the copyright and trademark rights to all the names from characters in the 80's, so I couldn't bring back anyone but those two."The confusion began to clear with the reveal of the Fluttershy toy prototype at the MLP Fair (image at top). While the pony retains Posey's yellow body and pink hair, it differs in being a pegasus with a purple butterfly cutie mark (hip symbol).
Lauren Faust also confirmed in the comments on this deviantArt page that the new yellow pegasus is not Posey, though is inspired by her colors.
"~plusleg1rl 4 days ago
...While a friend of mine was at Comic-con 2010, she spotted a yellow pegasus with Pink Hair. Is that Posey or a new character?
*fyre-flye 4 days ago
New character. But I used Posey's colors on purpose. She was a childhood fave."
Fluttershy is also the name of a G3 (2003-2009) pony, which also has a butterfly cutie mark. The G3 and G4 Fluttershys do not have similar hair or body colors though. Hasbro has previously reused pony names for ponies with different appearances; various G1 (1982-1995) names were also used for G3 ponies, including Bowtie, Moondancer, Starswirl & Minty.