What about the hardmode pets? I found the hamster in a gold chest after hardmode that i left unopened.
Not sure if that makes a difference either, the hamster may have always been there.
I know for a fact that you can only get the slime and bat pre-hardmode. Therefore the other 3 only appear in hardmode. Wiki.
None of what you just said is true :/ I'm almost certain I've found slime and bats in hard mode. Also, hamster was the first pet I ever got, it was in literally about the 5th chest or so that I ever opened, also I think it was in a brown chest near the surface (pre-hardmode btw).
So yeah, pretty sure all the pets can be gotten at anytime, hard mode or not.