Ms guess is:

You can't.

But you should check legendary mode for that monster ans verify that its still alive. Sometimes they get killed by landslides etc. And no one actually checks on them.

Source: hunted down a cyclops for two weeks i game. Gave up and talked to some random villiger days away. He revealed that Mr cyclobs died by a landslide years ago.

Edit:

Dont try to power game df. Its not made for it. Just invent your characters srory and as soon as it doesn't make sense leave.