気になるあいつは怪獣少年 (Ki ni Naru Aitsu wa Kaijuu Shounen)

Background
This song was uploaded on March 3rd, 2012, and has reached over 100,000 views in Nico Nico Douga, and on it's YouTube reprint, over 500,000.

It tells a story about a girl who had a crush on the new boy, and on the first day of summer break she followed him home. He went into a field, and she discoverd he had a pet monster. After promising not to tell anyone, they start having a relationship together. On the last day of summer break, the monster eats the boy. The boy tells the girl not to worry and he that he was still him. They go to school together, and everybody is screaming and freaking out. As the "sports meet" began, the two of them run in an "obstacle course", and people are firing at the target, the Monster Boy. The end is unclear as to whether the boy was shot down or not.