You can get mount Summer Wilderling Harness in Korthia during rare event. In chat you can see the announcement - "Escaped Wilderling roars defiantly in the distance" (for Night Fae covenant) You have to come to place noticed on your map and calm Wilderling down by riding it. After it you will have a chnce to get a mount (it has a low rate drop).
Here you can buy Summer Wilderling Harness for World of Warcraft with ReinwinBoost team for EU or US Servers.