You can change the horse's manes and tails at the horse stylists by Steve's Farm, Firgrove, Valedale and the West Cape Fishing Village. Sadly you can only change the length, but not the colour.
Each style costs 19 Star Coins.
When you change the mane, the tail changes automatically.
You can't change the manes of Frisians.
But you can change the manes of Andalusians and (if you want to) you can change it back to the originally mane. You can change the pony manes, too.
The first picture from the tails goes with the first picture of the manes and so on.