New laws could affect you

ALL dogs irrespective of breed or age need a Dog License which is free from the local Ayuntamiento.

Without a registration the fines are extremely heavy.
Spain has announced new laws No 50/99 which could affect you.  A list of dogs considered dangerous has been drawn up as follows:-

Pit Bull and all Bull breeds
All Mastiffs
Canary Island Prey Dog
Caucasian Shepherd
Bordeaux
Brazilian Fila
Akita
Doberman
Rottweiller
Corsican Can
German Shepherds.

Most dogs over 25 Kilos are included on the list.  For more information ask at your local town hall.

The new decree prohibits the training of dogs to attack, or to guard or defend their owners or to give them any sort of training that might increase their aggressiveness.  Fines vary from €2,400 to €15,025 Euros.  Dogs are banned from areas where children play.  A lost dog must be reported to the Police immediately. Dangerous dogs must be muzzled in public and on a short (one metre) lead.
All dogs that have any record of aggression or any dog that shows aggressive behaviour or any dog considered by the authorities to be dangerous, must now be registered and in possession of a special Licencia Municipal.

Spain is a federal country and has 17 different Comunidades or regions with their own parliament, government and laws.  A law in one Comunidad could be different from a similar law in another, therefore I strongly urge you to check with your local municipality.
In Valencia, Madrid, Barcelona and other large cities a list has been made of dogs which could potentially be dangerous, and of the Rottie, Doberman, Pit Bull Terrier, Dogo Argentino, Akita Dogo de Burdeos, Staffordshire, Presa Canaria, Fila Brasilero, are on the list, and also their crossbreeds.

Even if your dog is not one of those listed breeds, but has bitten a person in the past, it will be classified as if it was on the list.

If you want to own a dog which is on the dangerous list, you have to go and see a psychologist and do a test on a computer (not difficult) just the same as for driving a car.  It costs between €60 and €72.
If you are a Spanish resident you will have to apply for a document from the Ministry for the Interior, confirming that you do not have a criminal record.  If you are not a resident in Spain, you have to apply for such a document in your country of origin and have it translated into Spanish.  Take the originals with you to your local town hall