The rats and mice in this country are normally pretty small, so imagine stumbling across one as big as a cat.
That's what happened in Indonesia where scientists found a rat that weighs around 1.4 kg, five times as big as the ones that live in cities.
Neville Kemp, the man in charge of the study in Indonesia, said the rat is 55cm long and has long grey hair.
The rat is one of two brand new species of animal discovered in the remote jungle in Papua.
The scientists also found a very tiny animal in the jungle, called a long tailed pygmy possum.
A pygmy possum
The area where the animals were found has been called The Lost World by some experts because so few people have visited it.
A expedition in 2006 to the region also found lots of new species, including plants and butterflies.
Many other parts of the jungle in Indonesia have been destroyed because the trees are being cut down to be used in building, but this area is very difficult to get to, so isn't as under threat.