Jigglypuff made its anime debut in The Song of Jigglypuff. It mostly apeared when Ash used to journey with Brock and Misty.
Jigglypuff' are usually famous for their sweet voice, but this particular Jigglypuff has a problem with its voice, so Ash and Co. helped it to regain its voice but Misty didn't catch it so whenever it arrives or whenever it comes on before a stage, it tends to sing showing its voice but in the process puts everyone to sleep. In anger that no one listens to it, it often draws in everyone's faces (People and Pokémon alike). It followed the gang throughout Kanto all the way to Hoenn, but not seen afterwards. It is believed that it started on a journey to find the one whom listens to it. This journey is believed to have ended when the Gang encountered it one last time in Hoenn, where it found a Whismur that listened to the song all the way through. It is believed to be either continuing to pursue its journey to find more people to listen to its song all the way through or to be satisfied with finding someone who was finally able to listen to its song, either way it has not been seen since, but may return due to its popularity in multiple countries. It also still has Ash's marker that it uses as a microphone.
It reappeared in the Pikachu short airing along with the 15th Pokémon movie. There is also a chance that it will appear again in the XY series.