Mew.
Pokemon number 151 is Manaphy
manaphy you see it in the Pokemon manision.
Number 151 is manaphy
manapy
y
Pokemon #134: Finneon. Pokemon #151: Manaphy.
In yellow, there are only 151 pokemon, number 151 is mew.
On the National Pokedex, that is Mew.
The Sinnoh Pokedex only has 151 Pokemon in it.
number 151 is mew. If in the regional pokedex its manaphy, which can be found in a book on route 212 in the mansion.
In Pokemon platinum, the number 151 is Manaphy. You see him by going to the one mansion with the trophy Pokemon garden in it. In the room with the owner, you will see a book. Read the book to get him in your pokedex. Otherwise, you will need to have a Pokemon ranger event to get Manaphy.
151 is Manaphy and can be found by looking at the book in Mr. Backlot's room in the Pokemon mansion on route 212.