Read http://mlp.wikia.com/wiki/Cutie_marks
While the toys and plushes are somewhat infamous in often only including cutie marks on one side of the pony, the cutie marks in the show appear on both sides of the pony that has them. Like real ponies, though, the ponies in the show maintain bilateral symmetry, so the cutie mark viewed from one side of the pony will be the same as the one on the other side except that it will be flipped horizontally.
In terms of css, it means applying a transform: scaleX(-1) to the image.
One of the best examples of this is Rainbow Dash. Her cutie mark:
On her left side, her cutie mark has the 'jagged part' moving toward the right as you follow the bolt from top to bottom (toward her tail).
On her right side, her cutie mark has the 'jagged part' moving toward the left instead (but still toward her tail!)
This is something that's easy to overlook about the ponies, and many people will make fan art or drawings of the ponies without really noticing which version of the cutie mark should be used on which side.
If using the MLP wikia of a character as a reference, the image they show for a character's cutie mark is the one from the left side of the character.
Just knowing that the cutie mark is flipped from one side to the other is the key point. While the show itself sometimes gets them wrong, you can generally refer to the show to see which version is on which side. Here are the cutie marks of some of the main characters to help
| Character | Left-side cutie mark |
|---|---|
| Rainbow Dash |  |
| Twilight Sparkle |  |
| Applejack |  |
| Fluttershy |  |
| Rarity |  |
| Pinkie Pie |  |