Most plurals are formed by adding “s” or a variant on this. “cats” and “dogs” are two such words, as is “words”.
Some terms are composed of more than one word and hyphenated, and it is not the last word that is pluralised. The plural of “governor-general” is “governors-general”. (Yes, “governor-generals” can also be used.)
The issue: I was reading a novel recently and ran across the word “passersby”. That is the plural of “passerby”. Look at how the plural is formed.