There's a big chance that this isn't coming at all. People have always expected it because of Planet Coaster; but Planet Zoo started from a much earlier build of Planet Coasters' engine which has resulted in severe changes that make it incompatible with just "copy pasting" features from Coaster. One of these features was for instance billboards, they explained in one of the livestreams that that was a feature they basically had to do from scratch because the way Coaster did it wasn't going to work.3: (and one I’m really shocked no one has discussed)- is the inclusion of a thememaker’s toolkit in said update a la Planet Coaster, not necessarily for animals- but for things like props
Given the financial state of Frontier atm, I see very little reason as to why they would invest time, money and devs into a feature like this. Especially as the modding community has basically done it for them for free. The benefit of modded content is that it extends the life-time of the game and makes people buy the game even when support has dropped; so it was a smart move from them to do it for Planet Coaster as there basically wasn't a modding community back then. In this case, there is, so Frontier can benefit from that without having to invest in a tool; so why would they?