Patch 7.3 added a lot of new mounts to the game, and many of these are dropable mounts that are found on Argus. Most of them are tied to the rare spawns that you can loot once per day, so there’s no use in farming them multiple times per day. However, you do get loot once per day per character, so feel free to kill them with any alts you might have laying around.
A few more general guidelines for the rares before we get into the different mounts: they do spawn fairly quickly if they are up. Most rare spawns on Argus are up for between 3 and 12 hours before despawning for a certain amount of time, so don’t worry if you don’t manage to hit it before a group kills it; you will get a new chance in a few minutes. This also means that if a rare is not up within a few minutes, you better not spend your time waiting; instead do something else and check back in 3+ hours. The drop rate seems to be around 2.5 % for each daily kill, and there’s a total of 4 dropable mounts, so you will get a new once pretty often if you got a few alts that do the rotation.