Wu Xing Long is one of the seven Shadow Dragons and the second to be fought by Gokuu and Pan. He is the dragon of the Five-Star Dragon Ball and represents the element of electricity.

Wu Xing Long was born from the negative energy that resulted from the overuse of the Dragon Balls. He was specifically born from the wish used by Master Roshi to resurrect Gokuu so that Gokuu could fight against the invading Saiyans Nappa and Vegeta (both Gokuu and Wu Xing Long acknowledged that they are kind of like brothers).

(Source: Dragon Ball Wiki)