Pluralization can be one of the most irregular and therefore frustrating aspects of English for native and non-native speakers alike. Words that end in y tend to be difficult to classify: the rules differ depending on the letters that surround them; and sometimes an s is merely tacked onto the end of the word, whereas in other cases the entire ending is transformed, most often to -ies.
In the case of the word monkey, the first of the above two scenarios is true. Learners of English might have cause to believe that the ending of the word is transformed into monkies, based on any prior rules of pluralization to which they may have been exposed.
But most words ending in -ey simply add an s to become plural.
Key becomes keys
Trolley becomes trolleys
Journey becomes journeys
Ergo in a question of “monkies or monkeys,” we can apply the above rules and say:
Monkey becomes monkeys.
But how do we know this?
The “y” debate
The most confusing aspect of pluralizing words that end in ey is that the very last letter is still y. This invites a lack of clarity as to the appropriate placement of -ies, hence the “monkies vs. monkeys” disagreement.
The solution is largely one of aesthetics. A word ending in a lone y, like community or even something as simple as fly, would look strange with a lone s tacked on to the end: it is more visually appealing to fashion a new ending to accommodate the plurality. Communities and flies look better than communitys and flys.
Words ending in -ey have more substance to begin with, so an added s won’t look so odd. Think of chimneys, for example. So “monkeys” doesn’t need anything extra done to it.