Dredgehorn (frog), Maelstrom's Crown (jellyfish), and Jaded Drudger (horse jade thing) are all cosmetic mount skins that were limited time cash-shop items.
The Verdant Voyager (Crocodile) is actually supposed to be a caravan skin and not a mount. So the mount might be in-game achievable. Perhaps /u/IntrepidStudios can let us know. Is the Verdant Voyager shown at the stream mislabelled?