OOP constructor overloading

#include <iostream>
#include <sstream>

using namespace std;

class Pokemon
{
private:
    string name, type;
    int hp;
public:
    Pokemon();
    Pokemon(string name);
    Pokemon(string name, string type);
    Pokemon(string name, int hp);
    Pokemon(string name, string type, int hp);
    string toString();
};

Pokemon::Pokemon()
{
    name = "Pikachu";
    type = "Electric";
    hp = 60;
}

Pokemon::Pokemon(string name)
{
    this->name = name;
    type = "Water";
    hp = 60;
}

Pokemon::Pokemon(string name, string type)
{
    this->name = name;
    this->type = type;
    hp = 60;
}

Pokemon::Pokemon(string name, int hp)
{
    this->name = name;
    type = "Grass";
    this->hp = hp;
}

Pokemon::Pokemon(string name, string type, int hp)
{
    this->name = name;
    this->type = type;
    this->hp = hp;
}

string Pokemon::toString()
{
    stringstream ss;
    ss << "Name: " << name << " -->   Type: " << type << " -->   HP: " << hp;
    return ss.str();
}

int main()
{
    Pokemon myPokemon1;
    Pokemon myPokemon2("Buizel");
    Pokemon myPokemon3("Treecko", "Grass");
    Pokemon myPokemon4("Serperior", 120);
    Pokemon myPokemon5("Dragonite", "Dragon", 150);
    Pokemon myPokemon6("Latios", "Dragon", 130);

    cout << myPokemon1.toString() << endl;
    cout << myPokemon2.toString() << endl;
    cout << myPokemon3.toString() << endl;
    cout << myPokemon4.toString() << endl;
    cout << myPokemon5.toString() << endl;
    cout << myPokemon6.toString() << endl;

    return 0;
}