If you have Amazon Prime and/or Twitch Prime, you’re eligible for a mount and pet in-game. Currently, it looks like it was posted early and the webpages aren’t working. It should be available during the month of September. Below are links to the claim page and a forum post about the website not working.
The pages show 404 errors for me, but according to Bethesda, it should be working later today or soonish.
Edit: Bethesda fixed the pages, so it should be working now. If you don’t have Twitch Prime, but have Amazon Prime, you can activate Twitch Prime for Free, otherwise you can start the free Twitch Prime trial. You will be able to keep the mount/pet after the trial expires.