From Bulbapedia, the community-driven Pokémon encyclopedia.
The Pokérap is a series of songs that was made popular by the Pokémon anime. They are the English equivalent of the Can You Name All the Pokémon? songs.
The main purpose of the songs were to list all the known Pokémon of a specific generation. The original and Hoenn Pokéraps rhymed; the former had many Pokémon names that rhymed and the latter rhymed normal words together instead of Pokémon names. No rhymes were made in the Pokérap GS, however.
In the first two songs, the evolutionary families would be jumbled up, and sung in different lines or verses. In the Hoenn Pokérap, this rule was not followed; many evolutionary families were sung with two or three of their members back-to-back.
There are three Pokérap songs, one for each of the first three generations. Pokéraps have not been made from the fourth generation onward.