Certainly Chazal knew what monkeys were, they are mentioned in quite a few sugyas.
However, the concept of watching the monkeys in the zoo is thoroughly modern and didn’t exist more than 60 years ago,more or less. And so the issue of pregnant women seeing monkeys is a new circumstance and requires shikul hada’as.
Be that as it may, it is comforting that we have taken old superstitions and applied them to new circumstances. Old superstitions don’t die, it seems, they just modernize themselves.