The Fresian horse represents Neptune, the fourth and final gas giant, and the only other ice giant apart from Ouranos. It is named for the Roman god of the seas (who was also, it should be noted, the god of horses and horse racing), the brother of Jupiter and the Roman personification of the Greek god Poseidon. The astronomical symbol for Neptune rests in the upper-left corner, while the filigree and droplet patterns evoke both the content of the oceans, as well as the breakers and seashells.
This is the fourth and final in a series of pieces depicting our four gas giant planets as horses. For Neptune I chose the Fresian horse because they are probably the most beautiful horses ever, and as Neptune was the god of horses as well as the sea, I thought it only fitting to give his planet the best. Another fun piece of trivia: of the four, Neptune is the only horse who is pictured at a trot.
It has been suggested that I render Jupiter's Galilean moons, as well as Saturn's moon Titan, in a similar manner. However, this is all I have planned at the moment. All four of these pieces will be going to the MFF art show next weekend, so we'll see how they do there.
More useless trivia: this picture was colored almost entirely to the tune of Season 2 of Deep Space 9. So you can play the theme from that show in your head as you look at it.