Hey, are you drawing those ponies yourself, or are you using some program or thing on a different site to make them? Because you have to make sure that you aren't doing art theft, which can be a serious offense. And by serious offense, i'm meaning on Rescreatu and in real life legal issues.
If you are using some way to create them other than drawing them yourself, this may still be ok. You just have to check on that site and see if they allow you to use the art in any way that you want, which includes selling it. And if they do, you should state where you're making them, so it doesn't look like there is any sort of theft going on.
If you're drawing them yourself, then none of that applies. 8D