You go to Prof. Rowan's lab in Sandgem town and talk to him. Their should be a cut scene where Prof. Oak comes and you should get the national pokedex.
Chat with our AI personalities
No because it is a sinnoh Pokemon not a kanto, johto, or hoenn
You can trade, use the action replay for Pokemon Platinum, or actually catch it without the action replay. If you have completed the Sinnoh Pokedex, go and see Professor Rowan, and he will give you the National Pokedex, then you catch all the Pokemon throughout the land of Sinnoh from all the other regions due to an "outbreak". But, where to catch caterpie specifically, you need to go to Eterna Forest, west of Eterna City.
# 134 in the Sinnoh Pokedex is Finneon PS: if you want to catch it you can get it by fishing on Route 215 with a super rod
It's Aipom, and you can only catch Aipoms on Honey Trees in Groups 1&2.
The secret garden.. To access this you must complete the Sinnoh Pokedex by seeing all Sinnoh Region's Pokemon.. Then if you want to catch Shaymin you must have the event key item Oak's Letter...