So far for me:
Vitreous Stone Drake: 500+ kills, with the most early ones starting after getting Tier 11 and being able to solo the boss as protection.
Ashes of Alar: got it on my 30th kill
Swift Zulian Panther: 80th kill
Armored Razzashi Raptor: 100+ kills, no drop
Drake of the North Wind: 50 kills, no drop
Onyxia: 35 kills spread over 2 characters: nothing
Amber Primordial Direhorn: Took me 200 kills to get Jade / Slate direhorns, no sign of this color yet.
Fiery Warhorse (Karazhan): 2 years, 7 characters. Looted it with my rogue after hitting 85 and killing him just for fun for the first time.
Overall, i tend not to give up anyway on mounts so one day ill get em all