If you're looking for manga similar to Pupa, you might like these titles. All recommendations are made by Anime-Planet users like you!
The first Ajin appeared 17 years ago and quickly became the most important discovery to mankind. While they appear to be human, Ajin are rare, immortal beings that are feared by the populace. Only 46 are known to exist in the world, and most try to hide their existence, for capture means being subjected to brutal experiments that never end. Kei Nagai is a boy who dreams of becoming a doctor to heal his sister one day, but after he dies - and revives - in a car accident, he learns the terrifying truth that he too is an Ajin. Now, Kei finds himself on the run from both human and Ajin monsters that wish to do him harm, while struggling with his new, unwanted identity.
Ajin and Pupa share one unfortunate item in common: very disturbing, immortal-who-can't-die gore, complete with vivisections while awake, being eaten alive, etc. I found both to be equally as difficult to read for the same reason, but think the manga would appeal to the same audience.