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From this question: Why is 'aircrafts' bad English, while 'crafts' is okay?
What those words (like "fish") have in common is they have two different plural forms: for example, "fish" has "fish" and "fishes". "Craft" has "craft" and "crafts". Rather than I'm worried about how to use those different forms, I just want to know what those kind of words are called.
Nouns that have irregular plurals, such as fish, aircraft, mouse, man, are called "irregular nouns" or "irregular plural nouns". The phrase "irregular nouns" is more common.
As stated in this answer from EL&U, such nouns are referred to as irregular plurals.
