Did the original Bem have a manga that was based on? As far as I know, the 2019 BEM series is based in a late 60's anime of the same name. Did that anime have an original manga version? Or did the story start with the anime?
All Humanoid Monster Bem anime are original, but they also spawned manga adaptation.

The first Humanoid Monster Bem anime in 1968 got its manga adaptation and was serialized in Bokura monthly manga magazine until 1969, then reissued by Kodansha in 2012 and got its paperback edition in 2010.

The second anime in 2006 got its manga adaptation and was serialized in Weekly Young Jump and Mankaku, then got its tankobon version in 2007.

The third anime, BEM in 2019 got its manga adaptation and was serialized in Manga Park application.



The manga spin-off Humanoid Monster Bem RETURNS was the only original manga, which was serialized in Monthly Shonen Gangan since 1993 until 1995. The anime (and game) adaptation was planned in 1997 but got canceled, and thus leaving only the manga.



Source: Japanese Wikipedia