Hi J.H.
The words derived from Latin origins often have adopted a regular English and keep their original plural forms. They end in -us -um and -a (from the Latin gender)
Radius: Radiuses / Radii
Curriculum: Curriculums / Curricula
Antenna: Antennas / Antennae
Other animals - ( fish, deer) come to mind as being the same in singular and plural.
Also "series" has been mentioned before.
Regards,
A-
s