Well aren't you a barrel of laughs. Mimiron's Head might be more common than it used to be but if http://guildox.com/popular/mounts is even close to accurate, 'every second player' is a huge exaggeration.
Yes, the majority of so-called 'rare' mounts are easy enough to obtain nowadays by anyone who has the time to farm them. Technically that would also apply to the five world boss mounts since 'anyone can get them for free'. So I'm not sure what you have in mind by a 'rare' mount that is actually still obtainable - unless you think this thread should be for Gladiator mounts, the unobtainable ones from the BMAH and mounts people got way back when they actually were still 'rare'. Would make for a pretty boring thread, if so because funnily enough, those are so rare that practically no-one has them.