Wolves, lives and thieves are examples of irregular plural nouns. Although it may seem as if some nouns have irregular plurals which don't follow any rules, usually they do, only the rules they follow are very old!
With the examples of "wolves", "lives" and "thieves", the rule dictates that the f or -fe of "wolf", "life" and "thief" becomes a v when followed by s/-es. Other irregular plurals change the vowels instead. "Mouse" becomes "mice", for instance, "goose" becomes "geese", and "woman" becomes "women". Other words don't change at all, whether they are singular or plural, as in the case of "sheep" and "fish" (although "fishes" is acceptable where "sheepses" is not). Finally, English has a few words which retain very old plural forms, such as "child"/"children" and "ox"/"oxen". Some words, especially these familiar, domestic and farm-related, words, are not in a hurry to change to regular forms!
Play this quiz to see how well you know your irregular plural nouns.