You do have to own/ have owned all the kanto pokemons tho except for the legendary pokemons you just have to see them once (also Mewtwo, he is located in the Cerulean Cave)
In order to enter the Cerulean cave you have Buy the Club Card in Blackthorn city (wich will cost you 200.000 pokedollars) and get 240 pokemons registered as owned in your pokedex, when you have 240 pokemons registered as owned then just talk to the Prize giver in the club building.
He will give you a Medaillon wich you will need to get past the Dragonite in Dragon's Den (You have to fight the lvl 120 Dragonite in order to get past).
Then after you have done that find and beat Lance, be prepared because he is kinda hard.
When you beat him you have to talk to the Guard who blocks the Cerulean cave and then he will let you pass hope this was helpfull :Grin: