Question: why goku has a tail?
Answer: Goku's tail got cut for the first time by Puar, then got torn off by his grandfather, and finally was removed by Kami. ... Eventually, in GT, Goku had to fight Baby Vegeta and he lost because his kid body could not bear Super Saiyan 3. The Elder Kai was able to get Goku rescued, and then grew back his tail.