So, there used to be a great guide on tumblr somewhere about basic guidelines for creating an MLP OC, but it got taken down or deleted. So, I'm going to write one I can keep here for reference. Hope it helps!
For this guide, I will use two examples of other's OCs that I have drawn in the past.
This OC, reasonably simple and sensible. Let's call him Steve.
And this monstrosity, bleugughh *shudder.* We'll call her Izzy.
This is a very important part. Now, most of you know the Pony Creator by generalzoi? Well, since most people use that to create their ponies, I’ll talk about it. The Pony Creator has the same options for everyone that uses it. So, if you create a pony with it, there IS somepony out there, with same mane style, possibly similar colors.
Character creators are supposed to give you the idea to your character, not the final product. You make a base with it and spice it up with your own style. A good way to do that, is to take your pony made in the creator and draw it in your own style, changing some details to your liking. Even if you can’t draw you can make a description of the pony based on the one made with the Pony Creator: ”Her mane is like in the picture, but shorter and flipped to one side(…)”
If you aren’t using the Pony Creator, remember the basics: Don’t use blinding, clashing colors, and don’t make an over-accessorized, hyper-detailed pony. It is supposed to be something you will enjoy using and drawing, not something that will be difficult for even you to remember. It's not a sin to give your OC accessories, but keep it in the realm of possibility. Would the pony reasonably wear this stuff on a day to day basis? Is there a particular event or reason they wear what they do?
Take Steve, for example. He's a very basic pony with typical design, and aesthetic yellow and orange tones. Easy on the eyes and entirely feasible in the MLP world. Conversely, Izzy is striking shades of blue, purple and black which, while thematic, can be hard on the eyes. She sports leather boots, a starry cloak, and fairy wings, accessories that are far too busy and cluttered for someone seeing her for the first time. Anyone seeing her walk around ponyville would certainly be curious or concerned.
One last thing about accessories. Remember that you have to draw them. Every time. That also means that anyone who would potentially draw your OC will also have to take the extra time and effort to draw them. Most people don't mind drawing on an accessory or two, but if your OC has forty things with tiny detailing on them, the artist may instead decide to not draw the OC at all. (Izzy's boots took me almost as much time as drawing the rest of her entire being.)
Here is a valuable piece of advice: Think of the TALENT first, THEN name your pony and make a cutie mark.
For example, what if you name your pony after a Winter, snowy theme? Then you think the talent must be ice-related. So you make a, let’s say, ice crystal cutie mark. Now which talent would give you a ice crystal cutie mark? When people ask your pony’s talent, you won’t have an answer. Is your pony going to be an artist? A doctor? A Musician? After you choose one, THEN you make a cutie mark about it.
Also, limit yourself to ONE cutie mark. No pony ever has multiple (with the exception of those afflicted with Cutie Pox). If your OC has a mix of talents, think of a creative way to blend those talents into a single, sensible cutie mark.
Alicorn OCs are frowned upon. Why?
Alicorns are supposed to be special, they’re royalty and godly figures. A random Alicorn frolicking around Ponyville is unacceptable for most fans. Not to mention they’re kind of mary sue-ish. You either fly, use magic, or none. You can’t have both.
I’m not completely against Alicorn OCs, like if you have a GREAT reason to why your pony is an alicorn, then it’s perfectly fine. (And no, parents being a pegasus and unicorn is NOT a valid reason.)
Canon-OC relationships are a tricky subject to work with. I believe that’s a completely personal choice, as long as it ins’t exaggerated. My OC, Madrigal Harmony, for example, works as a secretary for Princess Luna. Because of that she’s an acquaintance to Luna, yes, but they’re not the SUPER KAWAII EPIC BFFS 4 LYFE. If your OC is greeeaaaat friends with everyone in Ponyville and everypony is crushing on him/her, they will be called Mary Sues.
Some people choose to have backstories for their OCs, which is perfectly normal. Make sure that your OC's backstory is something believable. It can have some unique or outlandish aspects to it, that's okay, everypony has unique experiences that make them who they are, but not everypony can be Twilight Sparkle and save the world four or five times a season. (Interestingly enough, Twilight is a really good example of how NOT to write an OC.)
These are some basics to abide by if you're looking to make a quality OC that you can easily blend into the world of My Little Pony. You don't have to follow them to the letter, of course, but hopefully some of these tips can help you avoid making a three horned, keyblade wielding, halo sporting, flame winged Princess Ohgodwhyyyyyyyy.