Nymphs can be found in many places, including forests, rivers, and even upon the seas. This variety also extends to their personalities, and views toward humans. You may find them friendly, or a fearsome enemy, so be wary when encountering them.
Still, they are generally not considered to be a threat to Rome, so rewards for their destruction will be minimal if you receive any at all. In some regions, all you'll get for slaying them is your own demise!