Argonaut is the Wilsons' robotic talking dog. He is an Advanced Robotic Guardian Operative (therefore standing for A.R.G.O.) created to protect his owners.
Prior to the events of the film, Rebecca and Cecil were unaware of his robotic nature until their house came under attack. Argonaut didn't bark and rarely ate, simply looking at Cecil when he threw food at him.
Spy Kids: All the Time in the World
He has long mechanical arms, oil spill, butt bombs, and has two modes: sentry or attack.