>>295389
Simple. Bring in one Echidna species and you'll have all the species for all the human males in the world in time.
For sake of speculation though, if Echidna species are not able (or allowed) to enter our world, then perhaps aiming for a monstergirl species that can use magic (such as witches, mages, liches etc) would be an alternative choice, to work out a way to bring in other species somehow. In addition to their magical abilities, they could monsterize human females into them; which could be ideal for their plans to take over and remodel the world - it's possibly far easier for monstergirls that 'matches' the human female as close as possible for the plan to success. I'm not sure on how to answer 'human civilization' question and 'worst girl' question though.