The English language is a, can't think of a word. I do know when i was at school fish was the plural of fish. Now everyone says fishes and it's acceptable. A bit like when I was at school there were 5 continents now there are 7.
Similarly. there used to be 9 planets.
The mnemonic to remember planets in order from the sun was :My Very Easy Method Just Sums Up Nine Planets where each initial represents a planet.
Last evening I watched the original Disney film Snow White and the 7 Dwarfs and wondered about the plural then 'cos I'd always thought that it should be dwarves.