The plural of "Rhombus" is "Rhombi". The plural of "Octopus" is "Octopodes". The plural of "Beef Wellington" is "Beeves Wellington". The plural of "Guinea Pig" is "Guineas Pig". The plural of "Rolf Harris" is "Rolves Harris". The plural of "Gortex Jacket" is "Gortices Jacket".