When buying mine, I figured out the following regarding different series. I hope it clarifies some of the confusion:
Between 2019 and 2021, they released Series 1 as silver, and gold proof, one animal every six months
Since 2021, they've been releasing Series 2 as silver, and gold proof, one animal every six months. So far 3 animals have been released.
Starting last year, they're releasing Series 1 as gold bullion, one animal every year [EDIT: That last sentence turned out to be wrong, see later in the thread, here]. So far just the elephant. Given the slower release rate, this is obviously going to keep falling further and further behind the silver/gold proof versions. That said, I found no indication if Series 2 will ever appear as bullion, or not.
I also saw, unhelpfully, some sites describing a coin for sale as from "Series 1" but with a picture from Series 2, or vice versa.
I haven't seen anything to say that there will be 5 different series, and I'm not sure where the 25 years mentioned in this thread came from?