Bokutachi Pichu Brothers - Party wa Oosawagi! no Maki

This is an animated cartoon that was released inside the game Pokémon Channel.

A half-hour long anime show, broken into five short segments, featuring the Pichu Brothers. Meowth throws a party that the Pichu brothers and their friends are not invited to. After a series of events, they find out they were invited, but their invitations were simply delivered late. They go to Meowth's Party and hear him sing. The series has a total of five episodes, each on its own channel.

(Source: Wikipedia)