A butterfly is a non-hostile, flying mob that spawns above the surface in purified temperate worlds.
Using a net, you can catch butterflies to later display them. Butterflies are one of only a few mobs that can be caught this way. (The others are scorpions and large roaches.) Successfully netting a butterfly will drop a mounted butterfly into your inventory that you can then place and display like any other item. For more information about these items, see the following pages:
Butterflies basically just flit around looking pretty and harmless. Amazingly, however, they are still able to physically push players around. No one understands why this is so, although it may be related to mutant superpowers resulting from over-exposure to apocalyptic radiation.
Butterflies are so easy to kill that you barely have to even touch them and they fall over dead. To kill a butterfly, just smack or shoot it with whatever tool you have on hand. One or two swings should do the trick.
Butterflies don't drop any goodies or give any XP when you kill them. But why would you want to kill a butterfly! Didn't you read the part about how they're so pretty and harmless? Bad butterfly killer! No XP for you!