Animal X: Aragami no Ichizoku

The first series in the "Animal X" timeline.

27-year-old Ayukawa Yuuji has never been much of a man; he's more comfortable sewing than fighting, he faints at the sight of blood, and his fiancee is tougher than he is. He's also a terrible pushover. So when his boss at the medical lab volunteers him for some experimental drug therapy, how can he possibly refuse? All's mediocre for Yuuji: master of the pathetic.

Mediocre, that is, until a juvenile mass murder suspect jumps out from under Yuuji's bed, declaring him his "mate." Before Yuuji has time to argue, the stranger transforms into a 16 foot-tall dinosaur and beheads his roommate...

Yuuji suddenly finds himself the sole target of a lost race that turns into dinosaurs, an extremely scary woman with a Goddess-Complex, the yakuza, a mad scientist and one 17 year-old boy who really really really wants to sleep with him.

(Source: Doki-Doki)