If you can remember that is.
Mine was a Crimson whelp. I was a little level 38 Night elf priest at the time biting my nails trying to figure out how i was going to scrape together enough gold to pay for my level 40 riding with each set of levels eating all of the money i had saved buying new spell ranks.
While i was sitting in stormwind thinking two players were talking about pets and how the Crimson whelp was really rare and sold for almost 150g
So i went all the way out to westfall and killed Little red whelps for days until i got one.
A member of the guild i was in offered to buy it from me enough to fund my level 40 riding and keep myself not hurting for water and spell upgrades.
But i just could not sell the damn thing and i still have it to this day.