Nate Crowley wrote for Rock Paper Shotgun about the broken (as of this writing) economy in Planet Zoo. The game allows players to raise animals and then sell them for money or the rare currency called conservation credits. But the economy is broken, and desirable animals are unobtainable to all but the early adopters:
In any case, the fact nobody is selling for cash has put CC at a massive premium: those who had got in early on endangered, prestige or hard-to-breed animals are now hoarding them, only selling them for wild sums. Prices have soared, making gorillas, tigers and the rest completely inaccessible to new players, while Pandas have become virtually mythical: beasts that might as well be made from pure diamond.
New players are forced to breed and sell just a few types of animals, like warthogs, to try to grind their way to a better future. As a result the market is flooded with low quality animals:
to make matters even more splendid, the animals people are selling on are the ones with genetic mutations that make them undesirable for further breeding – they might be highly susceptible to disease, tiny, incredibly short-lived, or even completely infertile. These sell for the lowest prices as well, naturally. Starting off as a new player, then, with your tiny pool of CC, you're going to be spending a lot of time scrolling through pages and pages of dying mutants, desperately seeking something that won't horrify your guests, and might have a chance of breeding.
You can read about efforts to fix the market here.