This may have been obvious to some folks, but it was not to me. Earlier in October there was a Twitch drop for "Call of the Wilds" to celebrate the launch of Rise of the Angry Earth. With this drop came a mount attachment.
The drop did not come over as a chest that had to be claimed nor do anything show up in player inventory. That is because the mount attachment is a skin and it may not be obvious to players, like me, that mount attachments have skins!
To use the Twitch drop skin open your mount equipment screen and then select the attachment slot you want to change. Right click the item you have there currently and pick change skin. This works just like changing skins for any other item slot.
|Right click > Change Skin|
|Call of the Wilds skin appears|