To better illustrate the Random choice solution:
import java.util.Random;
Random rand = new Random();
int percentage = 25; //Set this to whatever percentage you want.
public void onCreatureSpawn(CreatureSpawnEvent e) {
//This will generate a number between 0 and 99 (inclusive)
int generatedNumber = rand.nextInt(100);
//If the generated number is smaller than the percentage you chose, you have "success".
if(generatedNumber < percentage){
addCharacteristic(creature, characteristic);
}else{
//Do whatever you need to normally.
}
}