The Celestial Dragon is obtained through the Little Pet Shop achievement. The achievement requires 150 pets collected in your companion tab.

The Celestial Dragon is mailed in-game by

      From: Breanni
      Subject: Keep Collecting!


      Wow, you’re becoming quite the collector of the cute critters, aren’t you?

      I thought I’d write a little note and send a small gift to encourage your efforts. keep it up!

      — Breanni