He's a bone mouser. Even catches the occasional rat.

Are you sure that your first sentence should not be "He's a born mouser"?

Yes, I'm wrong. you right.
it's "born mouser"
sorry.

even - adverb

A2 used to show that something is

Thank you,

I've already seen that dictionary.
I think it's a etiquette to find dictionary first

And... I know meaning of 'even'
But in those context, I don't understand the word 'even'

I'm very beginner in English.
so I'm afraid that my question is rude or wrong...

Thank you for your help..

Does it help if we put in the subject of the verb?

"He's a born mouser. He even catches the occasional rat."

I understood what you said. Thank you!! ^_^