Hypnos' Blanket
Official Description

With the Hypnos' Blanket, you can bed your horse down outside of its box. It means that you can bed it down even if it is not registered in an equestrian center!

A Hypnos' Blanket is a Black Market item that can be acquired, be traded or found when defrosting the divine Frost.

This item enables a horse to be bedded down without having to be boarded in an Equestrian Center.