I know I'm going to get hate for asking this, why are many of the OC stories feature only human OCs or OCs solely based on creatures from the canon series instead of new creatures that the show hasn't featured before or yet? I'm sorry for making anyone mad or upset by asking this, but I just wanted to know why.

7906665
To be forward. I have no idea. All I know is that everyone wants themselves to be in another place making their owns stories. Both figuratively, and literally. I myself rather start an uprising in Equestria in my own story. Others want to write out their fantasies because they hate their everyday boring lives. Even some have weird kinks they want to share with the world. And those that made this site are allowing it to slide.

7906665
I don’t think such a simple question could make anybody mad.

I reckon the reason is pretty simple—fanfiction is a derivative material, and as such, people like to imagine new stories primarily within canon boundaries. Introducing a completely novel species of creatures is surely possible, but it requires more work regarding world-building and such. Plus, if you introduce a new character this way, then their differences may detract from the story at hand.

You know, if you wanted to write a detective story taking place in Canterlot, then having the detective be some sort of Eldritch horror monster and having nobody mind it would feel surreal. You could add that in as a random comedy element, sure, but if you were planning on writing a simple serious story, then a detective pony OC would be your best bet.

Moreover, many readers are interested in canon stories or at least canon-like stories, and writers may take that into account as well. That’s not to say that you should avoid adding some novel creatures—I myself did it with dragons that function differently from Equestria’s usual dragons because the plot demanded it—but it’s good to be aware of possible pitfalls.

I don't exactly know honestly. I admit I don't think I'd add anything to the conversation, but I guess I'm in a minority, since the species of my OC in my main story is a species not from the show. Pretty much all OCs I've made are either original species or creatures not from the show, with maybe one or two outliers.

I would say, if you want to write a character who is an original species, go for it, nobody can stop you.

7906665
A very good question, which I beleieve has already been answered adequately by my peers, here. But allow me to add my two cents in, as well.

The most obvious reason is because this is fanfiction. If this were regular fiction, then of course we could put in harpies and fishmen and mantids and all manner of other OCs It's certainly not difficult. But one of the great draws of fanfiction as opposed to regular fiction is the boundaries. I've had OCs that were gryphons, donkeys, a goat, dragons, a deer, a manticore... giant humanoid mantis men... But they've all fit into their particular pocket of fiction.

If your reason for asking the question is because you want to make up your own creature and put it in the story, I will stand in support of your decision to do so. I will also warn you that it really should make sense within the bounds of the story, which also must abide by at least some of the boundaries set by the series in order to be validated on this site.

Not to spill too much information on the subject, but I'm working on something with someone else where some of the main characters meet space aliens, so coming up with creatures is part of the magic. I suggested that the plant-based creatures could be subject to inosculation if they spend too long sitting in their chairs. Take your ship out for too long, and you find that you're all fused together. Fun!

I guess that's just what the majority of people are more interested in. Seems like cheating if a pony story has all humans, though, and they're not even EQGs or anthros.