The conversation between human and AI assistant.
Does everything imaginable either exist, did exist in the past or is going to exist?
Hmm, that's a good point. Maybe something like a unicorn?
Actually, unicorns did exist in mythology and folklore in the past, so they technically did exist in some sense. But to answer your original question, it's hard to say for sure whether everything imaginable necessarily must exist in some form or another. It's more of a philosophical debate than a scientific one.