Snakes are carnivorous reptiles that come in many varied species.
In Rio, a snake was seen when Blu and Jewel walked through the jungle of Rio de Janeiro at night, devouring a frog whole, scaring Blu at the same time. It played a minor role in the movie.
A snake named Ssssalbatore appears in the graphic novel prequel to Rio 2, written by Stefan Petrucha and James Silvani, Snakes Alive. One is a grass racer, while the other is a giant anaconda named Salbatore.
In Rio 2, Blu at one point accidentally flew into a boa constrictor's mouth, with Eduardo saving him by jumping on the snake. In the climax, the same boa constrictor ends up eating Big Boss.
It's possible that the snake seen in Rio 2 is bigger than the one seen in the first film, probably because the one is the first film is a tree snake, and the one in the sequel is a boa constrictor.