Here's what rhymes with megapotamicum.
                        
                        
                    
                                capsicum  Words that almost rhyme with megapotamicum
weaken  deacon  beacon  archdeacon  rerum  diesem  teatime  sheikdom  sheikhdom  freedom  hominum  cerium  chiefdom  threesome  fiefdom  museum  labium  cilium  samarium  mausoleum  imperium  encomium  trapezium  arboretum  solarium  francium  coliseum  protactinium