The arrested - two of them minors - allegedly beat the capybara with sticks and iron bars. The attack was captured on surveillance cameras, which led to the perpetrators being found.
"This is a brutal crime that shocks society," police chief Felipe Santoro told the daily O Globo.
"It is an act of extreme cruelty against a creature that posed absolutely no threat."
Capybaras are the world's largest rodents, weighing more than 130 pounds. The animals, which have become increasingly popular on social media in recent years, live in the wild in South and Central America and are often seen roaming Rio de Janeiro.
The injured capybara is now receiving treatment for head injuries, swelling, internal bleeding and injuries to its back.
