What kind of animal is Reepicheep?

He is a large, talking mouse who carries a rapier, and wears a red plume tucked in his golden circlet. He is an experienced warrior, utterly fearless, and faultlessly courteous, particularly to noble ladies. He is also pugnacious and quick to defend any affront to his honor.

Standard field mouse. In the Narnia books all the talking ones are larger than their wild counterparts.

a mouse