From Bulbapedia, the community-driven Pokémon encyclopedia.
The Pokérap is a series of songs that was made popular by the Pokémon anime. It is the English equivalent of the song Can You Name All the Pokémon?.
The main purpose of the song was to list all the known Pokémon of a specific generation. It usually rhymes, as heard in the original Pokérap which had many Pokémon names that rhymed, but no rhymes were made in the next one as there was a limit on names that rhymed and Pokémon. The third Pokérap had rhymes, but didn't rhyme any Pokémon, instead rhyming normal words together, or with the Pokémon's name.
Usually, the evolutionary families would be jumbled up, and sung in different lines or verse. In later raps, such as the Hoenn Pokérap, this rule was not followed; the Lotad family was sung together.
There are currently three Pokérap songs, one for each of the first three generations. Raps for the fourth and fifth generations of Pokémon have not yet been released, but are widely expected by fans.