At the middle of season 5 of My Little Pony: Friendship is Magic, Hasbro has released a partly censored S5 cast poster. The poster shows most of the cast of the season, with many characters hidden by a blue overlay. You can see this image here (please comment if you know a higher resolution version) and some description about it here. 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?
With some help, I have identified all ponies visible (not covered by the overlay) on the image other than these two.
Below is a modified version of the poster. 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, the two yellow ponies this question is about, 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 (wearing 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.
Update much later: by the end of the season, the community (but mostly Kinrah) has identified all the obscure characters too, so I'd better list them here for reference.
- On the left side of the poster, in front of Minuette, [A0] is the yellow colt with super-teeth and three carrots as a cutie mark, from E18.
- Next to him, [A1] is the bright red filly with super-strength and a hedge-hog cutie mark, from E18.
- Behind Minuette [B0] is an unknown pony.
- Behind Moondancer [B1] is Coco Pommel, from E16.
- Behind Twinkleshine [B2] is an unknown pony that may be difficult to identify because the poster doesn't show enough of its contour, but that may be Diamond Tiara's mother Spoiled Rich from E18.
- Behind and from the left of delegates are [B3] is Method Mare 4, the purple mare with red hair from E16, playing Charity's friend.
- Behind the two delegates [B4] is Method Mare 1, the green stallion with green hair, the one who plays the businesspony.
- In front of Twinkleshine [C1] is the bright green filly who Diamond accuses with “mane extension” in E18.
- Behind and to the left of Princess Cadence [D0] is Method Mare 2, the red mare with purple mane, who plays Charity Kindheart
- Behind Princess Cadence [D1..2] is Method Mare 3, the orange mare with pink mane, who plays the businesspony's assistant.
- In front of Princess Cadence [E0] is an unknown pony.
- In the center of the picture, in front of Rainbow Dash and Twilight [F0] is Scootaloo; in front of Pinkie Pie [F1] is Apple Bloom, and in front of Rarity [F4] is Sweetie Belle. They are cut from that photo taken at the end of E18, where two of them are showing off their new cutie marks.
- Between Rarity and Twilight [F2..3] is probably Starlight Glimmer.
- Between Shining Armor and Pinkie [G0] is Wind Rider from E15.
- On the right hand side of Pinkie [H0] is Orchard Blossom (actually Big Mac in a costume) from E17. This explains the absence of Big Mac, which used to baffle us.
- Behind Rarity and Fluttershy [H1] is Sassy Saddles.
- To the left of the Doctor [H3] is Incidental Pony (Whoa Nelly), the fat pony who asked for the Princess Dress in the end of E14.
- Behind Fluttershy and to the left of the Doctor [H2,H4] is one or more unknown characters, but the one in front of Cinnamon Chai [H2] might be Fashion Plate, the journalist from E14.
- In the back between the royal sisters and the griffons [I0] is the pony shaped waving balloon advertisment for the furniture shop from E19.
- On the right side of the picture, behind Double diamond and in front of the left speaker of Vinyl's table [J0] is Goth Pony (Moonlight Raven), the gray mare with purple mane from E14 who bought the "Over the Moon" dress inspired by Princess Luna, and next to him [J1] is her sister Bright Pony (Sunshine Smiles), the pink mare with yellow mane from E14 who bought the "Tripping the Light" dress inspired by Princess Celestia.
- Next to those two ponies and between Tree Hugger and Maud Pie [J2] is Limestone Pie from E20.
- In front of the Smooze are rest of the Pie family from E20. On the left [K0] is the mother Cloudy Quartz, in the middle [K2] is the father Igneous Rock, and on the right [K3] is Pinkie Pie's twin sister Marble Pie.
- On the balcony on the left [L] are eleven Hooffields from E23. In particular, the rightmost [L10] is the leader Ma Hooffield, and the second from the left [L1] is the ancestor Grub Hooffield from the story Fluttershy tells.
- On the balcony on the right [M] are eleven McColts from E23. The leftmost [M0] is the leader Big Daddy McColt.