People brand horses to signify ownership and/or what ranch they were bred at or came from. Its actually fun to try to find who owns the brand, the library usually has books of brands. Most brands are registered, especially if the same ranch runs cattle.
We bought a horse with a brand, I was able to trace his ownership to a ranch in Colorado. My mare is branded also, the man who bred her raised QH and brands all his foals.
Get a good closeup, or even shave that patch or hair and you will be able to see the brand better.