Rata vs. ratón
While both rata and ratón refer to rodents in Spanish, they are not interchangeable. The primary difference lies in the size of the animal, but they also carry distinct connotations, have different grammatical genders, and are used in unique idiomatic expressions.
Rata
A2A rata is a rat. This word refers to the larger species of rodent, often associated with sewers, filth, and disease. It is a feminine noun (la rata). Colloquially, it is also a very strong insult used to describe someone who is despicable, treacherous, or extremely stingy.
Vimos una rata enorme corriendo por las vías del metro.
(We saw a huge rat running on the subway tracks.)
Ese hombre es una rata, traicionó a todos sus amigos por dinero.
(That man is a rat, he betrayed all his friends for money.)
La plaga de ratas fue un problema de salud pública en la ciudad.
(The rat plague was a public health problem in the city.)
No seas rata y paga tu parte de la cena.
(Don't be a cheapskate and pay for your part of the dinner.)
Ratón
A1A ratón is a mouse. This word refers to the smaller species of rodent, which can be a house pest, a pet, or a character in stories. It is a masculine noun (el ratón). Importantly, ratón is also the standard word for a computer mouse. It has a much more neutral or even endearing connotation than rata.
El gato persigue al ratón por toda la casa.
(The cat chases the mouse all over the house.)
Necesito comprar un ratón nuevo para mi computadora.
(I need to buy a new mouse for my computer.)
Mi hija quiere un ratón blanco como mascota.
(My daughter wants a white mouse as a pet.)
Deja tu diente debajo de la almohada para el Ratón Pérez.
(Leave your tooth under the pillow for Pérez the Mouse (the Tooth Fairy).)
Summary
In essence, the main difference is size: a rata is a large rat, and a ratón is a small mouse. This distinction leads to different uses and connotations. Rata is overwhelmingly negative, referring to a pest or a strong insult for a vile or stingy person. Ratón is more neutral, referring to a smaller animal, a pet, or a computer mouse. Grammatically, rata is feminine (la rata) while ratón is masculine (el ratón).