There is a certain image released by Hasbro recently, which, if you look at it carefully, contains important spoiler hints about future episodes in season 5. A description of the image follows in the spoiler block, without showing the image.
The image a poster showing most of the cast of S5, with many characters hidden by a blue overlay. You can see this image at https://i.sstatic.net/YbqRB.jpg (please comment if you know a higher resolution version) and some spoilery description about it at http://www.equestriadaily.com/2015/07/new-season-5-poster-pops-up-with-hidden.html . It seems that all characters that don't appear in the first 13 episodes are covered by blue overlays, but some additional characters are covered as well. Nevertheless, we have identified some potential spoilers (that I won't list here) from even the outlines of the covered characters.
Here is a cropped and partly obscured part of that image, showing two yellow mares:
Who are these two yellow mares?
Two unlikely guesses for the earth pony among the two is Cherry Jubilee and
Adagio Dazzle.
With some help, I have identified all ponies visible on the image other than these two (but of course, I may have made a mistake or missed somepony).
I mean the ones that aren't covered by the blue overlays at least.
See a modified version of the image with labels, below identifications will refer to those labels, though they should be understandable without.
* On the left side of the crowd [N], The shark Vinyl's car jumped over, Lyra and Bon Bon, Minuette with Spike on her back, Moondancer, and behind them, Trouble Shoes and Party Favor.
* On the middle left part of the crowd [O], Twinkleshine and Lemon Hearts, Silver Spoon, Diamond Tiara, Cheerilee, and behind them, the delegate from Whinnyapolis and some other delegate.
* In the middle of the crowd [P], Pipsqueak, Tank, Applejack, Rainbow Dash, Twilight Sparkle, and behind them, Princess Cadence and Shining Armor with the wedding cake and that figure on top that's spawned some discussion, and Pinkie Pie.
* On the middle right part of the crowd [Q], Rarity, Fluttershy, Sugar Belle and Double Diamond, and behind them, two yellow ponies, one of which may be Cherry Jubilee, then the Doctor and Rose.
* On the right side of the crowd [R], Maud Pie, Tree Hugger, Vinyl (DJ-Pon) at his DJ car, Octavia, the Smooze, Zecora, Granny Pie, and Gummy.
* On the top right side of the picture [S] is Derpy (Muffins) sitting in the Doctor's ship.
* On the right hand side background [T], Steven Magnet holding Bulk Biceps, and Fluffy Clouds.
* Next to them [U] are the one-eyed monster depicted by the Idol of Boreas, Grampa Gruff, Greta, and Gilda.
* Near the entrance [V], Matilda and Cranky, Princess Celestia, and Princess Luna.
* On the left hand background [W] are the angry beast and the yak youngling Pinkie met near Yakyakistan, the yak Prince Rutherford and his two lieges (weraing party supplies).
* In the air on the extreme left [X] is Moon Glider.
* In the air to the left form the entrance [Y], the bugbear, Sunshower, Clear Skies, and Open Skies.
* Finally, [Z] is Discord, hovering in the middle. From the list of episode names, I believe he'll return in at least one more episode.
All of these are characters that appear somewhere in S5 E1 to E13, which gives us reason to believe that the two yellow mares are such ponies as well. The two mares may or may not be connected to each other.