Public trade
This is the easiest way to trade horses, go to the trade center place your horse up for trade for whatever payment you wish to receive.
You can then advertise that your horse is for sale in the trade center and other players can buy them straight from there for the correct price.
Private trade
This is a trade between two players using the friends feature, you can also use this to safely trade a horse to another one of your own players.

Trades can be for coins, credits or materials/feed/treats if you prefer.

Remember before you click submit on the trade that you have put the correct price as there is no way to change this once the trade is completed and asking a buyer for more coins is not acceptable.

Mods do not value horses - you decide the price for your horse and if someone wants the horse they will pay that price.