Added by Forestry.
Butterflies have a (very low) chance to randomly spawn from leaves with Forestry genome information (i.e. any treealyzed or bred tree). They are 99% decorative creatures, but will sometimes randomly cross-pollinate leaves. Bees are much faster though, and butterflies tend to randomly fly all over the world once spawned unless you keep them in an enclosed space (glass dome for example).
Pollen can be produced by an alveary with a sieve block. They allow you to manually cross-pollinate tree leaves by taking the pollen and rightclicking a leaf block.