No veo al perro.
What is the difference between using al instead of el in the sentence "no veo al perro."?
and the a is there because it's a "personal a". In this case, the dog is being treated like a person because it's a pet. "no veo el libro" versus "no veo al jefe".
This is because "al" is a contraction of "a" and "el". "No veo a el perro" sounds a bit strange, so it gets shortened to "No veo al perro" If the object of the sentence was feminine, there would be no contraction: "No veo a la gata"