Most likely it was to identify the month/year the colt was born...some cases its dam, like some do for cattle but usually they are accompanied by a ranch brand. Some wont put a ranch brand on a horse they don't want affiliated with their ranch/breeding program.(not saying that's your case WN at all)
Fire brands on a horse are hard to read sometimes because guys that brand cattle forget or don't realize how thin hided a horse is compared to cow and will leave it on. Then it scars and blotches.
If you want to register a brand contact your local ag dept. Office. They will give you guidlines on the size, what area on the animal and how many characters may be used. On most applications they want you to list your 1st, 2nd, 3rd choice in the event it is taken. Also you can look on some ag websites, people "sell" their brands for that state. Prices depend on the age of the brand and desirability.
Some brands depending on the size and formation have a tendency to blotch and not be very readable especially at a distance. My advice is to pick something easy to read and brand.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.