It's probably the sandy surface she's on, and the perspective effect that makes it look like underwater distortion and the greenish-blue background and plants that look like algae and the dragon snout which looks like some coral or big sponge thing or whatever. It only really looks underwater in the thumbnail though.
That's an awesome dragon's-eye view of Pinkie using her super special attack on it. It's great to see Pinkie so fiercely animated, showing a reactive and powerful side that is overlooked when she's in her normal everyday life. The nearly barren mountaintop and "angry" clouds add to the thrill, like the storm is about to rage as their epic battle begins. I can't tell you how wonderful the foreshortening is on the rubber chicken, and those feet look like they'd make a nasty slap if they hit.
The mountainous background and lovely valley, etc. are all wonderful too. But I'm more concerned for the red dragon's sake.
WOW. just WOW. At first I had thought this only took something like 4 hours, but after watching, I realize the perseverance and work it took to produce this image. What is ironic is that I like the grass the best, but those were what you spent the least time on doing.
You have incredible attention to detail, from the scales on the dragon's snout to the stray hairs on pinkie's mane to the forest down the mountains to the tiny changes within the iris of her eyes. everything is so detailed, it's simply incredible work.
I am left speechless. this is truly fantastic. Very well done, you have superb talent for this type of art