Some words can go either way. “Fish,” for example can mean more than one of the critters but you can also say “fishes,” particularly if you’re referring to more than one kind.
Here’s another critter that can go either way. The lady behind the counter gets the distinction.
- Use “shrimp” when you refer to the ocean-going crustacean. “I had a dozen shrimp for dinner.”
- Use “shrimps” when you’re referring to more than one small human. “We fourth-graders are shrimps compared to those seniors.”
Do any other double-form plurals come to mind?