IPA: //ˈrætənər//
KK: /ˈrætənər/
A person or animal that catches or hunts rats.
The ratter was skilled at finding and catching the pesky rodents in the barn.
The word 'rattener' is derived from 'rattle' (from Middle English *ratlen*, meaning to make a rapid succession of short, sharp sounds) and the suffix '-er' (meaning a person who). A rattener is a person or thing that makes a rattling sound.
Think of someone or something that makes a 'rattling' noise — the '-er' indicates a person or thing that performs that action.