Coincidentally, I just hit #500 last night myself (I have to head over to the being lucky thread ... Searing Scorchling
out of just my 2nd bag), and I felt a little disappointed that such a big threshold (essentially the biggest, roundest number you can get) doesn't do anything. Like the OP, I too am still not a Zookeeper ... but for me, it's because I didn't really start until February, and missed my chance for the Snowy Owl!
All of you folks who impatiently waited for Summer, try to imagine doing it all over again for next Winter.
Like all good overly obsessed collectors, I keep a table of my pets, and whenever I review it, I check the number of remaining "wanted" pets, and have to say ... the number of ones I can realistically earn without major luck is dwindling. I mean, I'll get the Kirin Tor Familiar eventually, I'm 165/250 towards the Direhorn and I'm pretty sure WoW will still be around by December 21st. But I have no control over how small a window my opportunities are for the ToT pets, never mind the virtual impossibility of earning a Darkmoon Rabbit now that the Faire is CRZ. I've never even seen an attempt to make a group, never mind a group, or a fight, or a drop.