This page explains in detail the interactions between Akira and South Park.
South Park episode: Trapper Keeper[]
November 15, 2000
Akira
South Park
When the Trapper Keeper fuses with Cartman, he becomes a gigantic blob with Cartman's features. The scene directly references a similar one in Akira, even included some shot-for-shot remakes of scenes from the anime film.
License[]
The creators of South Park held no rights to Akira, but only included a minor reference to the series.