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Selebi is also a real Pokémon, and not a mistake.

Like Mew it cannot be caught on Red, Blue, Yellow, Gold or Silver, but you can catch Mew and Selebi on Pokémon Crystal. (Great!!!)

Selebi is also very rare and can only be gotten on download events from Nintendo. Hope you have the luck to catch one.

Who is Selebi?

Selebi is the cute little Pokémon with the number 251. Selebi looks a bit like a little green ghost. It can travel through time and is said to be the guardian of a forest. Seleby looks after the forest, but as it is very rarely seen, the people living near the forest think a kind God is watching over the forest.
Selebi is the main character in the fourth Pokémon film, along with #245, Suicune, who is also a very important character in Pokémon Crystal.
As I said, I think Selebi can be caught in Pokémon Crystal, in the forest it is said to guard, after you've done a few certain things.

Catch Selebi

Again there aren't many ways to catch this Pokémon.

1. Trade him from a friend who has got him.
2. You can catch him in Pokémon Crystal.
3. You can win him on download events from Nintendo.
4. You can catch him with a Game Shark.

Different spellings

Seleby isn't very well known, and in the internet and in magazines you will probably find many different spelling. Here are the ones I have seen:

Celbi, Celebi, Celebii, Ceribi, Selebi, Serebi, Seribi, Seribi, Selibi, Seleby.

Rumours

Rumour 1

1. Catch all the Unknown forms in alphabetical order.
2. Go to the small house nearby and talk to the man at the end.
3. He will open a secret door for you.
4. Go in and you will find Selebi.

(That will take ages)

Rumour 2

1. Get all 250 Pokémon.
2. Now go to Blaine and fart in his sandwich.
3. Now a Selebi will pop up with a nose-peg and shake its hand.
4. You have now caught a Selebi.

(. . . . ?!)

Rumour 3

1. Get all the Elementary Stones.
2. Talk to the old man in Viridian who used to show you how to catch Pokémon.
3. Fly to Cinnabar and Surf on the coast on the left side.
4. You'll meet Missingno. He will NOT damage your game.
5. Catch him.
6. Take him to the breeding center and breed him with a Mew.
7. Wait for a long time and get the egg.
8. Walk around for 1,000,000 steps and it will hatch.
9. It has no attacks, so you have to teach him some TMs or HMs.

(Sounds like a lot of work.)

Rumour 4

1. You must have defeated the Elite Five and Red at least once.
2. Defeat the Elite Four twice and the Elite Five three three times.
3. Go to Red, walk around him twice, talk to him and he will give you Selebi.

Rumour 5

1. Get 200 Pokémon.
2. Put them in as many boxes as you can.
3. Start a new game and open box two.
4. Switch off and continue with your previous game.

(If you open another Box in a restarted game it'll overwrite the saved game, so this code won't work.)

Rumour 6

1. Go to Cianwood City and smash all the rocks (dont get into battle, or you'll have to do it all again.
2. Go to Goldenrod City and talk to every one.
3. Go to the Day Care Center and the man will be outside, even if you didn't give him any Pokémon.
4. Talk to him and he will give you an egg that will hatch into Selebi.

Rumour 7

1. Get a male Abra and a Female Oddish.
2. Train them both to Lv.30.
3. Put them in the Day Care Center.
4. Come back after about an hour and collect the egg.
5. After 3,000 steps the egg hatches into Selebi.
6. His attacks are: Psychic, Giga Drain, Psybeam, Solar Beam.

Rumour 8

1. Have all 16 Badges and all 250 Pokémon.
2. Get 6 shiny Pokémon and have them in your active six.
3. Beat the Elite Four and Red once.
4. Go to Oak and he will rate your Pokédex.
5. He will say: "(your name), you are the greatest Pokémon collector of all! Of course, you are still missing one Pokémon to make the complete Pokédex. Here take this. it will help you on your quest of completing the Pokédex."
6. He will give the G/S Ball.
7. Go to the shrine in Ilex Forest and put the G/S Ball inside the shrine.
8. Now you can catch your Lv.30 Selebi.

(I almost believe that one.)

Rumour 9

1. Go to the top of the lighthouse in Olivine City.
2. Talk to the Ampharos and use an Escape rope.
3. Heal your Pokémon 11 times.
4. Fly to Goldenrod City and talk to the man in the bike shop (he'll give you a skateboard).
5. Go to the breeding center and give the lady a female Mewtwo (from the old game) and give the man a male Mew.
6. Go outside, save the game and turn it off.
7. Come back the next day and the man will give you a Selebi egg.

(There is no such thing as a male or female Mew and Mewtwo.)

Rumour 10

1. Get the following team: Raikou, Suicune, Entei, Articuno, Zapdos, Moltres.
2. Put the following Pokémon on your PC: Mew, Mewtwo, Lugia and Ho-oh.
3. Beat the Elite Four three times and Red once.
4. Catch all 250 Pokémon.
5. Go to Professor Oak and let him rate your Pokédex.
6. He'll say: "You have completed your Pokédex, exept...that you are missing one Pokémon... Here, take this."
7. He gives you the G/S Ball.
8. Do the steps 7 and 8 of Rumour 8.

(It's almost the same as Rumour 8.)