What are words called that are plural not ending in s?

Nouns that form the plural by adding 's' or 'es' to the end of the Word are called regular nouns.

Nouns that form the plural in some other way are called irregular nouns or irregular plurals. Examples:

singular / plural

mouse / mice

man / men

woman / women

child / children

person / people

goose / geese

ox / oxen

foot / feet

tooth / teeth

cactus / cacti

alumnus / alumni

medium / media

knife / knives

wife / wives

baby / babies

memory / memories

alumnus / alumni

bacterium / bacteria