Bio Hunter

they both possess strong inner monster inside them.  

Both anime have something that is hidden within the human which emerge and leave a bizarre scene. Is it evil? I don't know. The main villain can sometime feel similar too. Bio Hunter is more mature and leave many questions unanswered but is worth watching if you are not looking for a great anime or a masterpiece. 

Both about people who have viruses who turn in to monsters. also same gore and violence.  

Humanity is under a threat of some of them being transformed into monsters. A duo of protagonists, one of them with the ability of controlled monster transformation and one of them in an advisor role tries to fight this predicament. Both titles are more adult oriented with violence, nudity and sex. 

Humans against monsters, and there's a person who standing between these two species. Even though he is not fully human, he is fighting the monsters to reach his goal Vampire Hunter D and Bio Hunter are classic anime that worth watching. The main difference between these two is Bio Hunter is a sci-fi anime meanwhile Vampire Hunter D is a supernatural anime 

Both involve battling with the "inner darkness/demon/beast" or whatever you wish to call it.  

In these two anime, the main character batte similar issues after becoming a monster. They both stuggle with doubting their humanity while battling against other monsters. These two anime are also very dark that are filled with blood, gore and nudity. 

both has the same supernatural theme,with an individual becoming a victim of supernatural power.The animation in both are great with lovely detailed drawing that give the charcters a real feel that you dont get with CG 

Both anime feature a duo of protagonist as they try to deal with demons that are possessing humans. However, at the same time, one of the main character is also possessed by a demon and must struggle to keep his human side, while battling other demons. Both anime have similar animation, and a similar dark and gritty tone. 

Both Pupa and Bio Hunter deal with a deadly virus that turns the main protagonist into a monstrous creature. The main protagonist in both anime kills people without being able to control themselves while trying to maintain a human form. Both anime are horror action-packed anime filled with violence, blood, and gore. The biggest difference between the two anime is that Bio Hunter has no incest. Not to mention Bio Hunter has better animation and execution. If you like one, you should probably like the other. 

The Guyver and Bio Hunter both feature a protagonist that can transform into a super strong humanoid being and kills numerous monsters with harsh violence. Both are action-packed anime featuring violence, blood, and gore. If you liked one, you'd like the other. 

Similiar style of animation. BioHunter has more of a story and doesn't feel like it's a segment of a longer movie like TotDM. 

Very similiar in story and character design. Animation is very good and gorey violence. 

Both anime feature a lot of blood, gore and sex (Urotsukidouji has more sex though). Both anime have a lot of scary demons with a very dark and gritty atmosphere. They are also horror action-packed anime with similar art and animation. If you enjoyed one, you will enjoy the other.  

Both anime are OVAs from the 1990's with a similar dark tone and atmosphere. The main protagonist in both anime can turn into a monstrous creature and kills people without being able to control themselves while trying to maintain a human form. Both anime are action-packed anime filled with violence, blood, and gore. If you like one, you should probably like the other.  

Both titles feature a protagonist that can transform into a super powerful humanoid being and battles various opponents filled with violence, blood, and gore. The Demons in Bio Hunter have monster designs similar to the Zoanoids of The Guyver franchise. Both are action-packed anime with a dark atmosphere. Differences between the two is that The Guyver is more sci-fi while Bio Hunter is more horror. If you liked one, you'd like the other. 

Both titles are horror action-packed anime with a similar dark tone and atmosphere. They both involve protagonists who are investigating a cause of an evil deadly virus that can turn people into monsters. Both anime are quite violent as well. The main difference between the two is that Bio Hunter has demons while Resident Evil: Damnation has zombies. If you enjoyed one, you'd probably enjoy the other. 

Both anime feature two protagonists that have to save humanity from an evil virus that can possess humans and turn them into evil deadly monsters. They are both action-packed anime with violence and blood. Both anime also have a similar dark and gloomy atmosphere. The biggest difference between the two is that Bio Hunter is way more violent and gorier than Black Bullet. If you like one, you may like the other.