They finally announced how you can unlock the free mount I mentioned in this thread.
Earn the Summerset Pathfinder Achievement and work with your fellow players to unlock up to three unique > Summerset-themed collectibles – the more players that unlock the Achievement, the greater the rewards for all!
SUMMERFALL IS HERE
From Thursday, September 20 at 11:00AM EDT until Friday, September 28 at 10:00AM EDT, work together with your fellow ESO: Summerset players to explore the island and earn up to three free unique collectibles, including the Psijic Mascot Pony pet, the Psijic Escort Charger mount, and the impressive Grand Psijic Villa house.
To help unlock these new rewards, you must acquire the Summerset Pathfinder Achievement by visiting all 10 of ESO: Summerset ’s “Striking Locales” located on both the home of the High Elves and the mystical island headquarters of the Psijic Order. Every day, we’ll update the Summerfall Event page to indicate the ESO community’s progress with a special meter. The pet will unlock at 33%, the mount at 66%, and finally the home at 100%. If you’ve already earned this Achievement, don’t worry, we’ve already included you in the tally.
You can track your Summerset Pathfinder Achievement progress and get tips on the locations from the Summerset section of your Achievement menu, but if you’re having trouble finding your way around, you can also find a helpful map below:
In order to be eligible to receive the rewards, you must have logged into the game while owning ESO: Summerset at least once (since its initial launch in June) by Sunday, October 7 at 8:00PM EDT. Note that rewards will be delivered by October 10, and even if you were unable to acquire the Summerset Pathfinder Achievement, as long as you have logged into the game while owning ESO: Summerset by October 7, you will receive the unlocked rewards for free. Don’t forget to thank your fellow players!
Finally, we’ll also have Twitch Drops enabled for all ESO Twitch stream! Starting on Thursday, September 20 at 10:00AM EDT, watch any ESO Stream on Twitch with a linked account for a chance to receive some Ouroborus Crown Crates. The Twitch Drops window will end on Thursday, September 27 at 7:00PM EDT. Drops will be awarded every 24 hours at 11:00AM EDT and are only available for PC/Mac users at this time.