Is it required that we draw the mane 6 or canon ponies? Or are OCs welcome?

It depends on the challenge!

Generally we accept ANY characters from MLP, canon or OCs as well.

But if the challenge itself calls for specifically Fluttershy or someone from the mane six or dragons only or whatnot then the entry needs to conform to that criteria.

However once that criteria is met OCs are welcome in such a case. Like, say you’re drawing Fluttershy. Once she’s there you can include the OC too if you want. That sort of thing.