Like many similar items, the FFXIV Hippo Cart Horn is a consumable item that permanently unlocks a mount for which ever character uses it. In this case you get the titular Hippo Cart.
You can purchase the Hippo Cart Horn with 18 “Arkasodara Pana,” which is the special currency given out by the Arkasodara Beast Tribe. The horn itself is sold by the NPC Ghanta in Thavnair. Though being able to purchase the horn requires you to reach “Sworn” reputation with the tribe first.
You can access the mount from then on, for free, from your collection. Note that this is tied to the specific character and not the account. You will need to purchase a duplicate on alts.
