I've updated the Warcraft Mounts page for 7.3. Please check out:
http://www.warcraftmounts.com/legion73.php
In addition to what Inay mentioned:
- there are a bunch of sources for things, but many are still listed as "Drop: Argus". I assume this means the notes are placeholders for various drops in the world of Argus and they don't all come from the raid boss!
- the Shackled Ur'zul is now blue-purple (was red). The other colours are unused.
- the Antoran Hounds now have sources. One is a boss drop, the other a raid meta-achievement reward.
- the talbuks are now Exalted purchases (were Revered) with Argussian Reach, except for the fel one which is listed as an Argus drop, but no defined source.
- the elekk seems to be definitely a paragon cache thing (with Army of the Light), but I'm not yet sure which colour is going to be used.
- one new colour of fathom dweller (jellyfish) mount on the way, this one from fishing.