"There is a bug in 1.10 version of the game. To complete this Achievement, you have to buy Saddlebags and Horse Blinders. They will be available for purchase from the Ofieri Merchant in the next patch."
Information provided by GamePressure (http://guides.gamepressure.com/thewitcher3/guide.asp?ID=3171...
), thanks to Hirogen D for pointing it out.
The information is not 100% but knowing CD Projekt Reds previous issue with the Gwent Achievement, I'm sure we can expect a fix for this Achievement in the next Patch.