I would 100% add shadows! Especially on the ground and on the horse because of the umbrella but other than that it's gorgeous!

I'm honestly going to disagree with what has been said haha! Correct me if I'm wrong but I believe the umbrella was a part of the original background image. It already casts a thin shadow backwards to the right. And you've copied the angle of that shadow really nicely with that of the horse. However, because it's such a steep angle, it makes it really hard to tell where the umbrella is casting shade. And the viewer's brain says that "oh a horse is under an umbrella so it should be in shadow" while in reality, the angle of the lighting is actually somewhat low and it's affecting where the shadow falls. So I think it's up to you as the artist to make a decisive call if the shadow is falling on the horse or not. If it's not, then you should make the horse significantly brighter just because the background is so harshly lit. If it's in shadow, I'd recommend making a decently sharp round shadow fall on the upper half of the horse. Consider that the shadow likely wouldn't reach all the way to the horse's legs due to the angle of the lighting and height of the umbrella.
Otherwise, I love the horse's expression and I think you perfectly captured that sunny beach nap. So cute!! ❤️

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Edit: You can see on the pole of the umbrella, about 3/4 of the way up, where the shade from the actual umbrella falls. Because of this, I do think a small portion of shade should be cast on the horse's rump and maybe some of the top line but not much more than that .

Thank you everyone!

Yes, the umbrella was there originally so I was try to mimic that shadow.

I didn't even see that shadow on the umbrella pole 😅

I will definitely work on lighting up to horse and adding that shadow!