The Spanish word 'ratón' (meaning 'mouse') comes from Medieval Latin 'rattus' (meaning 'rat'). It entered Spanish as 'rata' ('rat') and then was combined with the diminutive suffix '-ón' to create 'ratón', literally meaning 'little rat' - which came to specifically refer to a mouse in Spanish.

A closely related and simpler Spanish word is 'rata' meaning 'rat'. While 'rata' refers to the larger rodent, 'ratón' uses the diminutive suffix to refer to its smaller cousin, the mouse.

The English word 'rat' is etymologically related to 'ratón', as both ultimately derive from Medieval Latin 'rattus'. While English maintained the distinction between 'rat' and 'mouse' as separate words, Spanish created 'ratón' as a diminutive form of 'rata' to distinguish between the two rodents.