The animal that was successfully trained to tell the difference between Monet and Picasso was a sea lion called Ronan. She was trained by a team of scientists at the University of California Santa Cruz to identify various shapes and objects, including paintings by Monet and Picasso. Ronan was able to recognize the differences between the two artists' styles with an accuracy rate of up to 80%. This groundbreaking research sheds light on the cognitive abilities of sea lions and their potential for learning and problem-solving.