An Isabelo horse. Isabeloes are the lightest shade of Palominos. They were first heard of in history during the time of Queen Isabella (where the name come froms) of Spain. The famed Mustang, Cloud, is an Isabello.
Either look to infinity and then slowly bring your eyes back to normal convergence while looking at this, or cross your eyes, and then slowly bring then back to normal. If you find the right eye alignment, you will see a 3-d image appear above the screen.