Lemonade is the favorite to use, found at Dollyhair.com
Dollyhair is currently out of stock and will not be getting more in for about 6 months.
RestoreDoll has some Katsilk called Lemon Blond 14, which is an exact match for Lemonade at Dollyhair.
Failing that, Applejack has the exact same hair color as those listed above, and her mane and tail can be repurposed to use as
Pony's hair. If you do this, You will get a very short, show accurate mane. I take Applejack's mane and fold it in half to make the plug, and get a length that is acceptable and layered nice and shaggy like the show. I only need to trim the bangs and style the hair to lay flat.