we played one game and there is spell (i cannot rember exactly text) but with it you could claim any monster on board. Does it allow you to claim dunwitch horror creature? Or if you get "return all monsters from street to cup" does dunwitch horror removes too?

Anarin said:

No; spawn monsters can never be claimed as monster trophies. But you can defeat it using other means (such as flute of the Outer Gods, etc.). Also spawn monsters can't return to the cup, so effects from mythos cards that return all monsters from a street to the cup don't affect the D. Horror.