Before the new core, when you switched factions, some of the mounts weren't correctly converted. To the best of my knowledge, they are now. So those people haven't changed factions in a long time. But it's also possible that they can or could be purchased with coins, you should check yourself.
The Brewfest Ram drops in the Brewfest dungeon which can be played in October and it remains in your possession forever.
Swapped faction to Alliance couple of month's ago, got a kodo on my human.
Swapped back to Horde 3 weeks ago, got a swift white mecha strider on my orc.
It should not work like this and I'm not sure if you get the same mount everytime or if it adds new mounts if you change again.
About the Brewfest ram wowhead has this to say.
"If you completed the Brewfest Riding Rams quest in 2007, you are eligible to buy this mount from Brewfest vendors. You cannot purchase this mount otherwise."
So yeah, it was a 1 time event.
I remember getting it few years back, so I guess it up to warmane to keep "blizzlike" or make it available again, usually they prefer blizzlike though, I mean, if it's mean to be a special mount it should stay that way, but "thats just like, my opinion man"