Papelucho y el marciano (2007)

Papelucho, who has always been fascinated by aliens, decides to catch one and sets a trap. Amazingly it succeeds, and the lost Martian child Det ends up living through Papelucho, with hilarious consequences. The two build a spaceship and travel to Mars, only to escape back to Earth again after the Great Leader of Mars threatens to eat them. Det eventually takes over the body of Papelucho's dog, Choclo, but when Choclo's health is put at risk Papelucho has a difficult decision to make.