Welcome to the Original Character application board!  This is the proper place to submit your OCs for eligibility in the site roleplays.
All regular forum rules are to be followed here.  In addition, we have some rules and guidelines for posts in this specific section of the forum.

The Rules
When creating your character, we ask that you please keep the following rules in mind and adhere to them throughout the process.

Show Canon
The show itself has done a wonderful job of building a world full of all kinds of creatures and places. We ask that you stick to what the show has provided and avoid anything that may contradict or make inconsistencies with the established canon of the show (and, to a lesser extent, the comics)

Mary Sue
No. Simple enough. If you're unsure of what this could mean, I direct you here (apologies for linking to tvtropes, but they do a good job of covering all the bases). To keep it simple, the characters are usually poorly written and screw with the canon (see above rule) to make their back story fit among other things. Your character cannot be perfect or better than the canon characters of the show, whom will be used as examples of limitations within reason. This means that, no, you cannot be faster than Rainbow Dash nor can you be better at magic than Twilight (as examples).

Canon Characters
Yes, you can apply for one (please check the relevant thread(s) first and ensure that they are actually unclaimed). Please avoid making any major (and in some cases even minor) changes to their story and history. This applies to both when you're using the character or even if you're writing your own OC. While we will allow some changes, it'll require the forum to agree on these before they get implemented.

Profile Pictures
While not a requirement, we do encourage you to use a picture of the character (or even related to the character). This helps other RPers know who they're RPing with and what they look like (should they forget).

Inspiration
Should you draw upon other sources for ideas on creating your own OC, we ask you to keep a few things in mind. Just pulling a character from elsewhere and making them a pony does not make a good OC. Those characters were created and balanced for those worlds, and that combined with their own history would likely clash with what has been set up in Equestria. Making a few references to another character or setting in your OC is okay, but just remember that and the earlier rule above about the show's canon.

Inactive Character
While we don't want to stop you from sharing your creativity, we simply ask that you only create a number of characters that you can reasonably control and handle. If you enter an RP with someone and then forget to post for weeks at a time as you're busy with other characters, it's not very considerate on the player you have effectively trapped. While on the topic of inactive character, should a claimed Canon Character go unused for two months, the account will be deleted and the character put back up for grabs. This limit may change should the character be needed for something or a lot of people are pushing for its use. If you claim a canon character and believe you will be unable to use it for that length of time (through a holiday abroad or something) then alert an admin or mod so we're aware of this.

Throw Away Characters
As an extention of the above rule, and to avoid too many suddenly disappearing characters, please don't make a character that you don't plan on using for long. Should you do so, you'll be informed of breaking this rule, and we will, unfortunately, have to deny any applications you make for any of the LIMITED race characters. This is to ensure the limited number of them don't get filled with dead accounts. There are, however, exceptions to this rule. Should someone running an arc make character's specifically for it, they would get a free pass. However, if a player wants to make a character to participate specifically in an arc, this would be handled on a case by case basis. We just ask that you inform us in some way, either via PM or IRC, or even on the character sheet itself, should that be the case. If you have any other questions, again, just contact us.

The Procedure

Application
Post about your character.  What are they like?  What do they look like?  What makes them tick?
The proper format can be found here.  Some deviation is allowed provided enough information is included.

Examination and Critique
A moderator or admin will take a look at your application and judge whether or not it meets the site standards.  If it is found to be lacking (or even adequate), changes may be suggested to better align the character with what is required.  These suggestions could be for any reason from needing more depth of character to eliminating self-contradictory information to correcting canon contradiction.
Keep in mind that the mods are not your enemies.  Their wish is to keep the forum and role playing as high quality as possible.  Any changes they wish for you to make are intended for this purpose and not as attacks to your personal honor/dignity/whatever.
This step will persist until the mods agree that your character is fit for role playing. 
Your application will not be removed from the list unless you ask for it to be removed or go a lengthy period of time without responding regarding it.

Approval
If your OC makes it to this step, congratulations!  You may now role play with that character!  The application will be moved to the Accepted Characters board.
This does not indicate that you will be able to role play as different characters, however.  Every character you submit must pass the scrutiny of the mods before you may bring them in.