After more than ~100 days, I still haven't seen any new spider monkeys (the big black gorillas that drop silk).
They simply ceased to exist. Don't know if it's because I had killed them (100 days ago) or they simply are supposed to despawn during some season and never respawn.
Steps to Reproduce
Kill spider gorillas. Play for many days. Never see any gorillas again.
This issue has not been confirmed by a developer yet.