Here's why:
* "Bonito" as a type of fish: The opposite would be another type of fish, like a salmon or tuna.
* "Bonito" as a word meaning "pretty" or "beautiful": The opposite would be "ugly" or "unattractive".
* "Bonito" as a slang term for something good or excellent": The opposite would be "bad", "terrible", or "awful".
Can you give me some more information about how you're using the word "bonitos"? That will help me figure out what the opposite is!