This item is found during the progress of the Brewfest quests.
You also gain these with the racing ram buff, when participating in the Continuing to help out Brewfest event.
These quests are timed, and when the buff giving you the racing ram leaves you, so do the reins.
There is an apprentice at the brewfest area who you can ask for another set of reins, whether you have the racing ram buff or not. If you do not have the buff, these do nothing. If you get the buff, these will be replaced; you will still have one set.