Yellow Oleander
This is a Yellow Oleander (aka Be-still Tree or Luckynut). It is a non-native Hawaiian plant. It looks harmless enough, but apparently all parts of the plant are highly poisonous and contain toxic cardiac glycosides that can cause severe illness or death if ingested. Even the sap can cause skin irritation and burning the plant produces toxic smoke. I'm guessing the feral pigs and goats don't touch this one!