That's right, most of the time DLC comes out a week after the release of the original game. Most of the time because the developer or the publisher leave some content out of the game to release for extra money. So yes, I guess this is actual additional content. Also they wouldn't make much money of the animals (at least not from the gamers) because the Sun Bear and Mountain Bongo are free. Yes I'm still hoping the Xbox 360 gets some kind of support. Also did you now Frontier is trying to make indoor exhibits? And some small exhibits like enclosures for spiders and such? Also they are trying to make bird houses.
You can find the information here:
https://www.facebook.com/ZooTycoon360One?ref=hl
Look for the post made on the 6th of December 2013.
I also agree with you there, I get that the Xbox One version is the main focus, but they really shafted the 360 version of the game as well. I played the version on 360 first, then on One and there are some things that I don't really get like there being no online features. Though I can say from playing both that they are mostly the same game.
Anyway, I'm glad to hear about the additional content that's available now. I'm gonna see if I can download it in some way.