Legendary Pokémon and hard Pokémon to get Guide made by TornadoMudkip.

Also in this Guide will be some tips to get some Pokémon easy!
Please enjoy reading this Walktrough, I hope that it's useful!

NOTE FOR EVERY LEGENDARY POKÉMON: Save before you are going to catch one. If you are defeated or the Pokémon faints, you can try this again!

Bulbasaur, Charmander and Squirtle

You can choose one of these three Pokémon just at the beginning of this game. You'll get them from Professor Oak. The other two you can find anywhere else so make for sure the right choice.

Snorlax

From Snorlax are only two in the entire game available. One on Route 16 west of Celadon City and the other is on Route 10 east of Vermilion City. Once you got from Mr. Fuji the Poke Flute you can wake them up. They are Level 30 and keep both a Chesto Berry.

Tips to catch Snorlax:

~ Buy a lot of Great or Ultra Balls.
~ Have a Pokémon with the move Hypnosis or Thunder Wave. (Like Haunter or Pikachu.)

This Snorlax is just a troublemaker!
He/She can just use the move Rest, with this Snorlax will be full heal and asleep.
But the Snorlax has a Chesto Berry and with that Berry your Pokémon isn't asleep anymore.
If the Snorlax has low HP, use your Poké Balls until you have caught it.
For sure you must careful and if you don't give up you'll catch the Snorlax easy. (Lucky that this one isn't the hardest one to catch.)

Eevee and its Evolutions.

You can only get one Eevee in the whole game. You get a Level 25 Eevee in Celadon Mansion. Unfortunately, you must take the back entrance, through to the Pokémon Center to walk around. Eevee has in total 5 Evolutions. Use a Fire Stone, then it becomes a Flareon. Use a Thunderstone, then it becomes a Jolteon. Use a Water Stone, then it becomes a Vaporeon. The stones can you buy in Celadon Dept. For 2100 for one Stone.

But Eevee can evolve in Fire Red and Leaf Green NOT in Espeon and Umbreon. You should therefore trade Eevee to Ruby or Sapphire. Where do you ensure his/her happiness is good, and it's daytime, one Level to rise for Espeon, and at night a one Level to rise for Umbreon. But it may also be easier, because if you have Colosseum, Espeon and Umbreon are the starters and you can trade them to Pokémon Fire Red and Leaf Green.

Porygon and Porygon2

The strange Porygon is that you never encounter in the wild, but as often if you are at the Game Corner in Celadon City.  You have to pay for Porygon, 9999 coins in Fire Red and  Leaf Green 6500 coins. If you trade Porygon while holding an Upgrade, it evolves into Porygon2. For this you must have the National Dex.
You can get the National Dex if you are the new Champion of this Game.

Hitmonlee, Hitmonchan, Hitmontop and Tyrogue

In the Fighting Dojo in Saffron City you will receive after defeating some trainers, a Hitmonlee or Hitmonchan from Level 25.

Nice, but how do you come to the other, Hitmontop and Tyrogue?
I'll explain this now,

Make sure you have the National Dex and the Rainbow Pass. Go to Four Island and go to the Daycare and put your Hitmonlee or-chan with a Ditto in the Daycare. Make some eggs and hatch them out. These are Tyrogues. Tyrogue evolves at Level 20 Hitmonlee, top-of-chan. Which depends on the statistics that it has at that moment.
If the Attack is higher than the Defense becomes a Hitmonlee, if the Defense higher than the Attack then it becomes a Hitmonchan, and if the Attack is as high as the Defense becomes a Hitmontop.

The easiest way to determine the stats is the nature. A Lonely naturally good for Hitmonlee, or any other nature that Attack raises or lowers Defense. Relaxed for Hitmonchan. For Hitmontop another naturally with Attack and Defense remain untouched like Docile. Of course sometimes succeed not only with nature. (especially at Hitmontop.) Fortunately, you can also use Proteins to increase the Attack and Irons to increase Defense.

You can indeed get Hitmontop from Pokémon Colosseum.

Lapras

You can get a Lapras for the first time in Silph. Co. After beating your Rival you see a person who gives you a Lapras. He/She is at Level 25. Later, you can also catch a Lapras into Ice Cave.

Omanyte, Kabuto and Aerodactyl

In Mt. Moon you can have one Fossil of the two. If you choose one of them you make the right choice between Omanyte and Kabuto. The Helix Fossil is for Omanyte and the Doom Fossil is for Kabuto. Aerodactyl can generate from the Old Amber. You get it in the museum of Pewter City (Side entrance, use HM 1 Cut. )
Where can you generate that Fossils? On Cinnabar Island, in a building left from the Pokémon Center. Go to the third room and give your Fossils to the Professor. Go in and out of the room, and he gives you Aerodactyl and Omanyte or Kabuto, all are in Level 5.

Kangaskhan, Chansey, Tauros, Pinsir, Scyther and Dratini/Dragonair

These Pokemon are all rare, and you can catch them all into Safari Zone. But Safari Zone consists of several parts and you should not be too long in the wrong place.

~ Chansey is in Area 1 and 3.
~ Kangaskan is in Area 2.
~ Tauros is in Area 3.
~ Scyther is in Area 1 and 2, but you can also buy the Pokémon in the Game Corner. This is only in Fire Red. For Pinsir same applies, but then in Leaf Green.
~ Dratini and Dragonair can be found by using the Super Rod, they are hard to find but worth to catch because it's final Evolution is really strong: Dragonite. You can also buy Dratini in the Game Corner.

Now the Legendary Birds.

What do you need?
~ HM Surf (For all)
~ HM Strength (For Moltres)
~ A Pokémon with the move Hypnosis or Thunder Wave.
~ A lot of Poké Balls. (Like 30.)

Articuno

Articuno is in Seafoam Islands. He/She is at Level 50.
Articuno is an Ice/Flying type Pokémon, so the best use against this Bird is Fire and Electric.
If the Pokémon has not very much HP left, use your Poké Balls.
It's handy to use the move Hypnosis or Thunder Wave, with this it will be easier for you to catch.

Zapdos

Zapdos is in Power Plant. He/She is at Level 50.
Zapdos is an Electric/Flying Pokémon, so the best use against this Bird is Ice or Rock moves.

Moltres

Moltres is completely on top of Mt. Ember. He/She is at Level 50.
Moltres is a Fire/Flying Pokémon, so the best use against this Bird is Water and Rock.

Raikou, Entei, Suicune

Once you have the National Dex, one of these run through in Kanto at Level 50.
Did you have Bulbsaur, then you can catch Entei, if you chose Charmander, then it's Suicune, and if you chose Squirtle, then it's Raikou. So one of them runs through Kanto and if you are where the Pokémon currently is on the road, chances are that you meet him. If you go to another place, the Dog also goes further. The easiest way is to use your Master Ball, but there are also other ways.
Look at this Walktrough from me: https://www.vizzed.com/boards/thread.php?id=61645

You can also get all three from Colosseum.

Mewtwo

Once you have the National Dex, you can catch Mewtwo in the Cerulean Cave. (You also need to complete the Ruby/Sapphire Machine.)
He/She is at Level 70, so watch out. Mewtwo is a Psychic Type.

So can you walk:


If you are there, just use the same tricks that I gave by the Legendary Birds.

Unown

And at the very end of the game, on Seven Island, you can find Unown's. (You also need HM Strength.) You see a Puzzel, if you completed the Puzzel you  then proceed to the seven ruins. In each of them you can find one of the 28 Unown.
28? Yes! because there are '?' and '!' Unown. Below is what Unown you find in the ruins (Left to right.)

Ruins Unown:

Monean Chamber: ?, A
Liptoo Chamber: C, D, H, O, U
Weepth Chamber: E, I, N, S
Dilford Chamber: J, L, P, Q, R
Scufib Chamber: F, G, K, T, Y
Rixy Chamber: B, M, V, W, X
Viapois Chamber: !, Z

Some are very rare, so keep looking at them if you want them all.

So I hope that this could help you a bit. (This isn't my best Guide ever.)
Thanks for reading my Walktrough/Guide!

Great job on this tornado. Once again, I find your walkthroughs very throughout and very useful when playing a game. I think that you did a great job of making a guide for this game, these always come in very useful and you put in 110% effort every time you write one. This is amazing so keep up the awesome work. You did such an excellent job of writing these, I know they will come to help a lot of people and make it easier for them to play the game. Your guides are awesome, keep it up and keep making more useful walkthrough guides! This is going to come in very useful!

So, This is the post you went for the Red Jersey, I see?

Amazing! This is long and detailed, and I would rate it a perfect score! This would help every one! Thanks for making this thread!

This is a really great guide, but you forgot t say one crucial thing, and that is to always save before you attempt to capture a rare Pokemon.

epic-san : Yeah, I saw, I edited my Walktrough for that!

Anyway thanks everyone for reading and enjoying my Guide/Walktrough.