There are a vast number of butterfly-themed apps in the Play Store (try searching for "butterfly" in there and you'll see). I imagine that many of them can produce notifications. Sadly that means that you will be very lucky if anyone can recognise this particular icon: as is often the case, if you really want to know the simplest way would be to ask the person whose phone you saw it on (these questions usually concern icons on someone else's phone, so I'm assuming that yours does too).