Pokemon that need to be obtained BEFORE reaching a certain point

[Jolteon0]

What pokemon and important items need to be gotten before a certain point in the game?

[Alistair]

That question is waaay too broad to be answered in a simple and comprehensive way. The main point of no return in the current game is the Agate City checkpoint, which you cross after obtaining the 9th badge. Typically you'd want to go on the hunt for missing mons and items right before reaching that point.

Go check the pinned threads in the On the Hunt section, they're massively helpful.

Also do note that no particular mon or item that isn't given to you through the normal course of the story is necessary to progress. If you miss some stuff, you can always go back after beating E15.

[Jolteon0]

Thanks!

[Tartar]

The only missable event at the moment is the Mystery Egg event where you get one of 19 rare Pokémon randomly, after saving the five police officers. But if you don't do the quest before finishing the area, you can't get the Mystery Egg.

But otherwise, every other Pokémon is not missable. Sure you may be locked out of an area for a while, but at the end, you will be able to return there and get whatever you might have missed. So don't worry about getting everything, and just enjoy the game.

[Via]

Typically a Pokemon will be made available later on if you miss it.

For example, in the Lower Peridot Ward there's a house with Whismur in it. After beating the first gym, the Whismur will be gone and can get an Onix instead.

However Loudred is available later on, and I don't think there's an "expiration date" for that event.