This is Flame, the Island Stallion. He is a character in several books of Walter Farley's The Black Stallion Series. This is a really good series about the bonds between boys and their horses. Most of them are about Alec and the Black, but some feature Steve and Flame or Tom and Bonfire. There have been two great movies and one pretty good movie based on the series. Also, a pretty good TV show. Walter Farley wrote the first book while he was in college, and continued writing Black Stallion books until he died. He made it his life's work, and took the reader on many adventures, from deserted islands, to the Middle East, to the racetracks of America, to the Everglades, to the western plains of the United States, to Spain, well you get the idea. 

The books featuring Flame as the main horse are 

The Island Stallion

The Island Stallion's Fury

The Island Stallion Races

The books featuring Flame alongside the Black are

The Black Stallion and Flame

The Black Stallion Challenged!