There are a variety of points within the game where you are given a Pokémon. These Pokémon for the most part take your ID and Number but are yours to keep. You need to have a space in your party in order to receive these Pokémon.
How to unlock
Enter The Dreamyard, you will find a lady who will gladly give you one of the apes. The Pokémon that you receive is, however, not up to you. Instead, you get the Pokémon of the type that is weak against your starter. If you chose Snivy, you'll get Panpour. If you chose Tepig, you'd get Pansage and if you chose Oshawott, you'd get Pansear. They are at Level 10.
To get Zorua, you will need to have first transferred Celebi over to Black & White using the Relocator Machine unlocked in Castelia City. Once done, take Celebi to one of the buildings in Castelia City where you'll see a girl with another girl who is apparently lost. Celebi will come out from its Pokéball and the lost girl will shed her illusion and turn out to be Zorua. Zorua will be offered to join you. However, you do need a Pokéball as one will be used up when you obtain it. It can be any ball.
Found in Relic Castle, receive from backpack guy, called "Cover Fossil". Bring to Nacrene City's museum and speak to one of the clerks at the entrance
Found in Relic Castle, receive from backpack guy, called "Plume Fossil". Bring to Nacrene City's museum and speak to one of the clerks at the entrance
Found at Route 18. When you enter this house, talk to the guy in the red suit and hat. He will offer to give you a Pokémon Egg. If you accept and hatch the egg, you'll discover that it's a Larvesta.
On the Marvelous Bridge, you will find a man who will sell you a rare Pokémon for 500 PokéDollars. Unlike in Red & Blue however, the Pokémon he sells you is rare and can't be obtained anywhere else in Unova
Found in the Ice Rock room of Twist Mountain, received from Worker. Bring to Nacrene City's museum and speak to one of the clerks at the entrance