Voyage Collar

This cushioned leather and rope collar is easily adjustable to fit the necks of all manner of creatures. You can place the collar on a willing Medium or smaller creature using an action. If the creature is a friendly beast companion or familiar (such as one summoned by the find familiar spell), you can speak the collar's command word as part of this action to create a magical bond between you and the creature. The bond remains until the collar is removed.

You can use a bonus action to speak the collar's command word while you can see the bonded creature within 30 feet of you. When you do, the creature grows in size to become Large, provided there's enough space. The enlarged creature gains a +2 bonus to AC (provided it isn't wearing armor), its speed increases by 10 feet, and is magically equipped with a bridle, reins, and saddle. The enlarged creature can understand the directions of any friendly creature that's riding it, and its carrying capacity magically becomes the same as a riding horse. The creature remains enlarged for up to 8 hours, until it drops to 0 hit points, or until you end the effect using a bonus action. Once this property of the collar has been used, it can't be used again until the next dawn.

If the beast or familiar wearing the collar is a transformed humanoid or other creature, such as by the polymorph spell or similar magic, the bond fails when you speak the command word and the collar has no effect. If the bonded creature is a familiar summoned by the find familiar spell, the collar and familiar disappear into a pocket dimension together whenever you temporarily dismiss it. If the bonded familiar is dismissed forever or drops to 0 hit points, it leaves the collar behind. A creature can only gain the benefits of one magical collar at a time.