I´ve seen a good amount of horseback riders near the coast and sometimes on the beaches around Sopelana(Sopela) and Plencia(Plentzia), towns outside of Bilbao in Bizkaia. I saw people riding in the Spring, when there are less people out, so I don´t know if they do it in the summer too. I don´t know much about it, but I can´t imagine a more beatiful place to go horse-back riding.