Super Saiyan 5 is a transformation the only Goku has shown so far while fighting Pikkon. It is shown that Goku doesn't have full control of the form as he keeps on transforming in to it while sparring with Pikkon. He later transforms into this form to combat Zaiko.
Zaiko has a false super saiyan 5 form which is due to his altered genes. The form greatly increases his speed, strength, and stamina.