Mutational Plurals

In English, most nouns are pluralised with -s or -es. Some are irregular, like child - children, wolf - wolves or cactus - cacti. Some stay the same, like sheep. However, in all of these cases it is the end of the word, if anything, that changes, while the beginning and middle stay the same. Which English nouns are not like this - in that the middle of the word rather than the end changes for the plural?