Mount collecting is the longest grind in WoW because most of it is drop chance, not skill. This shelf holds the whole collection: the Midnight season mounts from raids, keys, delves and PvP, and the classic rares people have farmed for a decade — Invincible, Mimiron’s Head, the Time-Lost Proto-Drake, Rivendare’s Deathcharger and the rest.
Every mount is farmed by hand on your character, so it lands in your collection and stays there.
Season mounts or legacy rares?
Season mounts are on a timer — once Midnight’s season ends, the achievement and PvP mounts stop being obtainable. Legacy rares are not going anywhere, but they are pure luck, which is why they are sold as a run count instead of a single kill.