This model has the same color and socks as the Dapple Bay (from the Dapples sets), but with no dapples.
Set #720076 Mini Whinnie Adventure Vehicle & Two Horse Trailer set from retailers such as Target in 2007.
Set #300125 Sunshine State Dressage Festival Play Set from 2007-2008, included the Loping Stock Horse, Walking Arabian, Rider, Dressage Arena and Play Mat. On the 2008 Retired List. These models tend to have more yellow tones and a lighter head. I have not seen enough of these to determine if this is a specific variation from the Trailer set.