Very thrilled that the Giant Anteater and the Jaguar no longer have to be part of my list. Nonetheless, the top 5 at the moment would be:
1. Maned Wolf - a Brazilian icon (and personally, an animal I adore), I was hopeful it could find itself in the South American DLC and am saddened by the fact that I don't think I'll see it any time soon. Still, a guy can't help but dream.
2. Red Kangaroo - I think this is on most everybody's list. Self-explanatory, I guess.
3. Southern Cassowary - I know everyone is saying emu. And, deep down, I know emu makes more sense. Still, I find the Southern Cassowary so much more interesting and a part of me will continue to hope it makes itself onto a soon-to-be revealed Australian pack.
4. Serval/Ocelot/Caracal/insert smaller cat species here - I understand the desire for putting big cats in the game: they're awesome and they're impressive. And trust me, I'd love to have an African Leopard in-game just as much as the next guy. Nonetheless, I feel like there's a whole roster of smaller cats that are just as interesting and that, presently, constitute a missing niche within the current species we can acquire. Would love to see one of these guys soon.
5. North American Beaver - I was rather surprised when it wasn't found in the original roster, as a player of the original Zoo Tycoon 2, I wanted to have the opportunity to build these critters an exhibit. I do believe, however, that it won't take long for this one to come out - hopefully with dam-building animations embedded.