When Jeremy Feasel (Muffinus) first mentioned on Twitter that there would be a bunch of new rare spawns with 100% drop mounts with varying degrees of rarity, I just knew that the Sunhide Gronnling was going to be the rarest of them all. Subjectively, I think the coolness factor of the other mounts also lines up with their relative rarity. The talbuk, elekk, and boar mounts seem to be the most common. Next up is the riverbeast and clefthoof mounts, which both seem to have respawns of roughly 24 hours.
Then we have the gronn mount. The funny thing is, people were beginning to wonder if Poundfist was just bugged and not spawning at all. Why? The first time it dropped on live servers was on November 23 — 10 days after launch! Why did it take so long? Because apparently its respawn timer is reset back to zero every time the server is restarted. There were too many rolling restarts, realm crashes, and other problems associated with launch that prevented Poundfist from spawning.