what mod that added butterflies? It seems i can't find any on my SSP worlds after wondering the whole place
Forestry does. They spawn from leaves of trees that have a Forestry genome of some sort. And no naturally generated tree has that.
You need to grab some vanilla saplings and run them through a treealyzer (or get a multifarm arboretum to plant them, that analyzes them too). Those trees will then have genome information, and once they grow you will start seeing the occasional butterfly. They are a fairly rare occurence though and will attempt to avoid players if they get close.