I think would be nice to have those, specially the furry caterpillars that would be very dangerous in melee approach, and the Monarch Butterfly that is very common. The Monarch could feature a monarch's armor to increase a pet/minion number (I don't know how the pet feature works, I didn't got one yet... But if the pet helps you, this could be great). But I'm thinking in this armor's perk just because the butterfly's name, no scientific stuff.
This feature could provide items from the cocoon to, but destroy a butterfly cocoon can be a bit uncomfortable, it would feels like when you have to kill a Ladybug, very sad situation, I hate when I need to kill the passive creatures.
I agree with you, big bosses are awesome! Anyone who've played Shadow of the Colossus will agree with that!