Translation:The cat sees a mouse.
"mys" wouldn't be "myse" here, wouldn't mouse have to be in the accusative?
myš is a feminine noun, and feminine nounds ending in a consonant don't change in the accusative case.
You can check it's declension here: https://en.wiktionary.org/wiki/my%C5%A1#Czech
smoaking, what are you smoking?