Bubbles is the monkey pet of the North kai, better known as King Kai. He serves a special purpose during King Kai's training lessons, particularly one of two endurance tests, where trainees are instructed to chase after Bubbles in an effort to catch him. This forces the trainee to push themselves in gravitation 10 times that of the Earth. Gokuu spends 40 days working on this test, while Piccolo, who is later killed by the Saiyans Nappa and Vegeta and is sent to train with King Kai in Other World, manages to catch Bubbles almost effortlessly (as during the year Gokuu spends running Snake Way and training with King Kai, Piccolo has been training both in solitude and with Gohan). This causes King Kai to outburst that Piccolo should know Bubbles' test was not meant for him, but for Yamcha, Tenshinhan and Chiaotzu.


(Source: Dragon Ball Wiki)