The swallowtail butterfly is a tiny, non-hostile mob found only in the skies of purified temperate worlds. You can capture one with a net -- high agility skill and a good net increase your chances of a successful capture.
All butterflies are extremely easy to kill, but who would want to kill the pretty little butterfly? Bad butterfly-killer, no kill drop or XP for you.
Related item: Swallowtail Butterfly (mounted)