You've got a start, though in my opinion it still needs quite a bit of work.
When you're writing something purely piano it's a tad strange to have the volume fade in. It makes it painfully clear that it's not a human performing it and, at least to me, detracts from the experience. While it's true that many of us just don't have access to a keyboard, taking steps to keep it from sounding too artificial will be good.
As much as I dislike using the term "should" when it comes to art, I really do think it would be a benefit to take a look at
this page. It details four parameters that pianists use to convey emotion, all of which are replicable in a DAW. Some of this is due to how the piano VSTI you've used isn't particularly great but what Levitin says will certainly still be helpful.
Add some dynamics. That will do a lot to help keep things varied. You already tweak the volume at the beginning and end but there's a difference between that and implementing dynamics, especially since the rest of it is at the same or similar velocity.
The left hand (aka the melody) sounds a bit harsher than the feeling that you appear to be going for. Just lightening the velocity ought to help with this. As for the harmony, try to vary it much more. It's very static and, frankly, not all that engaging. There are many, many types of harmony that you could implement that it's a shame to just repeat block chords.
I'm not really getting a pony sense from this. That might just be personal taste, but to me it doesn't seem like this has much to do with ponies. I mean, it's just called "Song of the Pony". Is there a specific pony you had in mind? That's too vague for a title, in my opinion, or at least it is without some kind of context.
The ending was also pretty strange to me. The left hand faded into silence while the right hand, if anything, seemed to become louder. It sounds like you lowered the volume when gradually reducing the velocity would have been more suitable.
How much music theory do you know, if you don't mind me asking? From this it doesn't seem like an overwhelming amount, but I don't want to make assumptions. I'm stating the obvious here, but the more you know the better your composition will be. If you're not that well-versed yet then learning it will take time but it'll absolutely be worth it.
Oh, and welcome to the forum! I hope my criticism wasn't harsh; I didn't mean it to be.