One of the boy in my neighbourhood has a monkey
One of the boys in my neighbourhood has a monkey.
The second one. Why? It’s simple, really, because when you add the word “of” in this sentence, what you mean is, there are many boys in your neighbourhood and one of them has a monkey. So the word following the “of” must always, always be in plural. For example,
One of the cars he has is red. (He has many cars, of which one is red)
One of the hikers was found dead (Of the many hikers, one was found dead)
One of the guests complained he was thirsty (Of the many guests, only one was thirsty).
So when you talk about one thing or person of many, the many is always a plural. Always! And because you are talking about that one thing, the verb that you insert in the sentence is always singular:
One of the cars he has is red (You are talking about that one car, not all of them)
One of the hikers was found dead (Just one was found dead, not all)
One of the guests complained that he was thirsty (Just one guest complained, not everyone did).
So there! I am really hoping this is one of the things you will remember the next time you write or say something about one of many.