Snakes are quite widespread in Spain, but you are probably unlikely to see one as they stay well away from humans. Many are flattened on the roads but they are a good indicator of a healthy and diverse countryside. All snakes are legally protected and beneficial to man (they consume large quantities of rodents and other agricultural pests). The only snakes of any danger to humans or pets are vipers which have a painful venom, although hardly ever fatal to humans. Vipers are smallish snakes with a patchy distribution in Spain. Even where common they are difficult to come across as they are shy and retiring. Of the estimated 50 snakebite deaths a year in Europe, only 3-6 occur in Spain, so don't worry too much. More people die from bee and wasp stings.
Montpellier snake (can get quite large) 2-metre long Montpellier snake. It is blue with a white underbelly -don't go picking one up to check- and has prominent ridges over the eyes.
Ladder snake - a species of Rat Snake found in Iberia and Mediterranean France. The young have a bold ladder pattern, which is reduced to two simple stripes in adults. Recently a two headed Ladder snake was found near Alicante - are rare creature now in residence at valencia University!
Lataste's viper - grey, short (around 50cm) and is distinguished by its triangular head and the zigzag pattern on its back.