Each rare mob can be killed once per warfront cycle, and they reset when your faction captures the Warfront zone. This means that you have a chance to get the mounts every 2 weeks (roughly), and each rare mob has a 10 % chance to drop the mount. So the drop rate is not that terrible, especially if you got 4-5 level 120 alts already.
There are a total of 6 rare mobs that drop mounts, but two of these are faction specific, so you can only get five mounts if you only play a single faction. Anyway, below is a map of all the rare mobs that drop the mounts, and make sure to read below if you want more information about each rare mob.
How to find Beastrider Kama - drops Swift Albino RaptorBeastrider Kama is located in Witherbark Village, and is guarded by two adds. This rare mob is pretty easily soloable, and drop the Swift Albino Raptor for either faction.
How to find Skullripper - drops Skullripper
Skullripper is a big raptor that drops a mount called Skullripper (I guess he drops himself?). This is the only saddle-less raptor in the entire game, so it's a very cool mount. Skullripper requires a group of 4-5 people to kill, and it will be very difficult to kill him alone. He is found roaming around north of Go'Shek Farm.
How to find Nimar the Slayer - drops Witherbark Direwing
Nimar the Slayer is roaming around in the Witherbark Village, and has a chance to drop the Witherbark Direwing upon being killed. He is soloable.
How to find Overseer Krix - drops Lil' Donkey
Overseer Krix is a bit different from all the rare mobs listed above, because he changes his location depending on which faction controls the Warfront. He drops the same mount (Lil' Donkey) independently of which faction controls the zone, and he can even be killed by both factions at the same time.
When the Horde controls the Warfront zone, he is located in the Alliance mines, and vice versa. He is soloable, but a bit difficult if you don't learn to dodge his attacks.
How to find Knight-Captain Aldrin - drops Broken Highland Mustang (Horde only)
Knight-Captain Aldrin is the Horde-only rare mob that will only be avaliable when the Horde controls the Warfront zone. He is friendly towards the Alliance, so only Horde players can kill him and loot the Broken Highland Mustang he drops. This rare mob is soloable, and you can find him at Dabyrie's Farmstead.