Demon Beast Invasion

Demon Beast Invasion (妖獣教室, Yōjū Kyōshitsu) is a manga series written and illustrated by Toshio Maeda, published by Wani Magazine. The series was adapted into a six-part original video animation of the same name in 1990, followed by Demon Beast Resurrection (妖獣教室外伝, Yōjū Kyōshitsu Gaiden) in 1995.

Plot
Earth is being invaded by a race of unclean beasts who wish to take over the planet. However, since Earth's environment is hostile to the demons they hatch a plan to impregnate as many human females as possible in order to create a race of human/demon hybrids which will rule the Earth by proxy. But the "Interplanetary Mutual Observation Agency" sends a duo of enforcers to Earth to hunt down and destroy the alien race they call the ‘demon beasts’.