Drone Coordinator was the least popular I could find
honorable mention to Trilomite, second least popular I could find.
other low popularity mobs off the top of my head: Calypsocod, Mourner, Nexnecis, Gradivore, Morsusilex, Faucervix, Igni/Diripi/Pupugi, Boorum Male/Female, Razortooth, Plumatergus, Tripudion