Rato vs. camundongo vs. ratazana
In Portuguese, while rato is a general term for mouse or rat, camundongo and ratazana are more specific. The main difference between them lies in size and connotation, with camundongo being a small mouse and ratazana being a large, often feared, rat.
Rato
A2Rato is the most general and widely used term. It can mean either mouse or rat and is often used when the specific size or type is unknown or unimportant. It is the default word. It can also refer to a computer mouse.
O gato perseguiu o rato pela cozinha.
(The cat chased the rat/mouse through the kitchen.)
O rato roeu a roupa do rei de Roma.
(The rat gnawed the clothes of the king of Rome.)
Meu computador precisa de um rato novo com fio.
(My computer needs a new wired mouse.)
Acho que tem um rato vivendo no porão.
(I think there is a rat living in the basement.)
Camundongo
B1Camundongo specifically refers to a mouse, a very small rodent. It is often used for house mice, pet mice, or laboratory mice and generally has a less negative connotation than ratazana.
O pequeno camundongo comeu o queijo na ratoeira.
(The little mouse ate the cheese in the mousetrap.)
Muitos experimentos científicos são feitos com camundongos.
(Many scientific experiments are done with mice.)
Minha filha quer um camundongo branco como animal de estimação.
(My daughter wants a white mouse as a pet.)
Um camundongo é muito menor que uma ratazana.
(A mouse is much smaller than a rat.)
Ratazana
B1Ratazana specifically means rat, and it refers to the large, robust type of rodent commonly associated with sewers, garbage, and filth. This word carries a strong negative connotation.
Uma enorme ratazana saiu correndo do bueiro.
(A huge rat came running out of the manhole.)
As ratazanas podem ser um grande problema de saúde pública nas cidades.
(Rats can be a major public health problem in cities.)
O barulho na lixeira era de uma ratazana procurando comida.
(The noise in the dumpster was from a rat looking for food.)
Ele tem pavor de ratazanas desde que viu uma na garagem.
(He has a dread of rats ever since he saw one in the garage.)
Summary
In summary, use camundongo for a small mouse, ratazana for a large, imposing rat (usually with a negative sense), and rato as a general, all-purpose term when the distinction is not needed or when referring to a computer mouse. Think of it as a scale of size and perception: camundongo (small/neutral) < rato (general/medium) < ratazana (large/negative).