Unfortunately the pets are not available yet.
But soon
Only real reason to actually collect this pet is once the promo is over .. the pet is no longer available.
Also once the pet is released one will need to visit the Mt. Dew site on a DAILY basis to get fuel (stack of 5). It is believed the fuel will allow the pet to "battle" a pet of the opposing faction (or different colored fuel anyway). Once fuel runs out no one really knows what happens. They may add the fuel as a in game item you can purchase or at the very least the pet will at least be able to follow your around.
The fuel and pet are 100% free and ONLY require one to set up a Mt.Dew account. You will also need to link your WoW account to a Battle.net account to get access to the code that will get you the pet.