English nouns whose plural form differs from singular

Solution 1:

From the Wikipedia article on suppletion:

In linguistics and etymology, suppletion is traditionally understood as the use of one word as the inflected form of another word when the two words are not cognate. For those learning a language, suppletive forms will be seen as "irregular" or even "highly irregular".

It gives both cow/cattle and person/people as examples, though person/people is “incomplete suppletion”. Suppletion covers more than just nouns though, including verbs like be/am/is/was/were, so the term you are specifically looking for is suppletive plurals (example).