Yeah, that's tragedy. That's why it is often called „ogród zoologiczny”. The Polish alternative (sadly not used) is „zwierzyniec”.
Człowiek człowiekowi wilkiem, a yeti yeti yeti/kiwi kiwi kiwi.
(man is a wolf to man, and yeti is a yeti to yeti/kiwi is a kiwi to kiwi)
For the word „zombi(e)” there is a dilemma: "Zombi zombiemu zombim" or "Zombi zombi zombi"
Zoo? Yes, there are many foreign words that don't decline ending in o. But, if it were not a foreign word, it wouldn't decline anyway