Percy jackson to find a dragon with diamonds are located in the neck, which is to kill it to get the diamond then he must remove the diamond, which is omitted in the pegasus horse, that is his weakness.
Percy jackson traveled as a dragon with a diamond in the neck to kill it, then, that diamonds are diamonds that he had to put on a horse pegasus was his weakness.
Percy Jackson went looking for the dragon with diamonds in the neck, which will kill it to diamonds, which he must put the diamond in a horse Pegasus is weak.