You just have to define that Animal implements Comparable<Animal> i.e. public class Animal implements Comparable<Animal>. And then you have to implement the compareTo(Animal other) method that way you like it.

public int compareTo(Animal other) {
    return, other.year_discovered);

Using this implementation of compareTo, animals with a higher year_discovered will get ordered higher. I hope you get the idea of Comparable and compareTo with this example.