Some nouns have several plurals. One of them eventually dies off, but it confuses children (and adult) until it does.
Some of them like “fishes” are hotly debated for centuries. Scientist love the word “fishes” but we as teachers usually use the word “fish” when talking about the plural of fish. Can you say the word “fishes”? I can’t! If you turn on the discovery channel all they talk about are “fishes.”