Looks like a lot of opinions regarding this subject, seems I should weigh in as well.
There is two points in regarding every aspect of an OC: firstly the OC should be original, It's in the title. Secondly the OC should be understandable for the setting they're in. With a few small exceptions in a CS this should bee straight forward. I'm just gonna go through each part and explain to the best of my abilities what I mean,
Name: this is pretty easy, if the name is owned and it's obvious to find then be careful not to use it. for example. Trixie Lulamoon; Taken, Dusty Roads; Taken. Taking a name is sometimes okay but only when the name is generic enough for example Mint Leaf. There is like 4 Mint Leafs. As for the character aspect of this make sure the name fits the setting. It can't be stressed enough how many times this is broken for a self insert. Under normal circumstances I.E. some kind of HIE RP or something, But pony names normally consist of 2 words. Be it a common noun, an adjective, or even a conjunction. It's traditionally two words, Now with that in mind one words, are okay but those characters might be harder to Identify going further down the road.
Personality/history: I clump these two together because they go hand in hand far too much to ignore. An Original aspect of this is that the OC's history and personality isn't directly similar to another character's, What this means is that R63 would fail this portion in the originality section if they attempted to pass it outside of AU setting ((Best R63 watch Sliders episode double cross)) But it still doesn't count. As far as character wise, there is three things that should be said: 1.) the character's personality should match up with what their history says ((No miserable backstory for a character that is constantly happy, and on the flip side no happy backstory for an edgy character... they don't match))
2.) The history is believable to the setting. ((Ponies are not traveling to human NYC like it's a piece of cake.))
3.) The Personality and history match up to the world around them as a whole. (What I mean by this is that in a world where everyone is all honkey Dorey and crime is almost nonexistent I doubt you're going to find a serial killer or a master criminal kingpin)
Appearance: This is the holy grail of making or breaking an OC, An original OC description is unique for that character. For an RP with humanoid characters this is a lot easier to accomplish since for the most part you'll be hard pressed to find two people that look exactly alike and not be twins ((I've seen a few doppelgangers before but that's aside the point.)) Humans have a lot of unique features that make them them. Ponies however do not, remove the mane and silhouette them you would not be able to tell with 100% accuracy which pony is which. ((Small exceptions aside)) So things like face-claims are unacceptable. And recolors can be accomplished by using a coloring book. So neither is really acceptable in the original setting. Generators can work but need to be good ones. I'd suggest General Zoi's pony creator V3 since it has the most options, It may look generic but with that you can get your foot in the door at making an OC that is your's. Outside those two you have artists renditions and bases, Both of these require more work but you can make an OC image that looks amazing.
For those that don't want images you can describe your character in words. Be warned avoid "Looks like"'s these generally make the reader visualize the pony mentioned so saying the OC looks like Twilight the reader is already imagining Twilight and all your work is just added to her like some kind of Chronenberg monstrosity. Instead try descriptors of their attributes ((Big mac's size, Pinkie's mane, Rarity's eye's.)) this can give the reader a composite to think off of. Kind of sloppy but can work. Last part on descriptions, If you can write details, While there is such thing as too much, It's generally loved for there to be a lot of detail.
I think that about covers it. All in all the main point of having an OC is that it's your character and it's unique, just having an OC for the sake of having an OC is a slippery slope that should be avoided since that removes any sense of personalization to the character and just makes them a thing.. Instead an OC should be an extension of yourself. Something you love to play in not some skin suit that you feel obligated to push around in a story because that's how OC's die quickly.