What's realistic depends on the size of their art team, the reusability of their assets, the skill and speed of their technical artists and animators, and the ease of implementation of new animal AI. It could be very simple to implement a new species, or it could be very complex. Basic scenery DLC is mostly new art on existing code, so fairly simple. The existing DLC animals were probably fairly straight forward art swaps, though I haven't played with them myself to verify; the Thomson's Gazelle is much like a Springbok, a Pygmy Hippo much like a Hippo, a Komodo Dragon like a Nile Monitor. Shouldn't have really needed to add much in the way of new animal behaviors or animations, just tweak the art and habitat preferences. Now that many of the rigs, base meshes, animations, materials, and code exist, they don't have to make much from scratch. How long it takes to make a new species game ready will depend on how much it differs from existing species morphologically and behaviorally. A polar bear will need a new mesh and everything from about shoulders up should get an animation polish pass if you use re-targeted animations from the brown bear. The fur material is good to go, just needs a recolor. A penguin on the other hand, would call for a model, a whole new rig, animation set, their feather material, and swimming and diving AI. It's realistic to say we could get a handful of animals by Christmas, depending on which species are chosen and the size of the team working on new content. To put a new tiger subspecies in the game should just be just a few variable tweaks, nearly effortless. To get a penguin or otter running believably right now would take additional code, and the coders are probably busy fixing more important things.