Yea, sorry I couldn't really explain in-detail. Basically, the event is during the night (not sure if weather is a factor) where there is a Hiker Guy who talks about really wanting a place to have. After you talk to him, there is a building in which a guy is talking about renting out the place and if you know anyone who wants to live there. Then, you go back to the Hiker and tell him about the place so he can live there. When you do that, he'll thank you by giving you a sweet Onix.
Here's some topics about it you can use: Onyx? and Found Onix early in the game (I think so...)