Barry was worried he would turn into the animal creature again when the full moon showed it's face. It happened every full moon, which was roughly a couple of times a month. So pretty frequent. It happened a lot. "Tonight you will not change." He told his reflection in the mirror. Intently.
He said that every time though. It still happened. He'd grow paws and canine looking teeth and hair. He also got bubbly bumps on his back.
He also would stay at friends' houses and say "Don't let me out of this room!" then he'd turn into the man animal creature beast and massacre his friends and smash their home. Then he'd wake up back to normal and wouldn't remember. No one in this town and world had heard of man-animal-morphing-at-moon-time-beasts. It wasn't a thing that pop culture had lore about in this story, so there were no special bullets or tricks known to stop him. Well there was one special trick where if you pricked him with a plastic sewing needle dipped in nail polish remover, but there were no mystical gypsy ladies or psychics to tell everyone about it. Plus even if you knew, you couldn't get close enough to prick the Changebeast without him shredding you to slop.
It was a small town with limited resources. Otherwise you'd think they'd have paid attention to all the murders and massacres happening.
One night, Barry had a lady he'd been dating over to sleep at his house. He didn't pay attention to what moon it was gonna be. Because people often just go on with their lives and only notice the moon is full when they look up and see "Oh it's full tonight, pretty." So Barry forgot then he massacred the nice girl he was seeing.
He also massacred cops. Because he was fast, they never caught him. One time the cops cuffed him but he snapped the cuffs, then ripped the cops who cuffed him's flesh to ribbons. He got away. Security cameras caught a lot of the horrible incidents on camera, but Barry didn't fit the description of the Changebeast he changed into. So he got away with it. He isolated himself inside his home by getting a bunch of cinder blocks and building a wall inside his room where he was real cramped. But then he woke up the morning and found that everyone in his town was dead. He had killed them all.
"Oh shoot." he said with extreme and unbelievable sadness. He was so sad about it.
He walked and walked and walked and went to another town. Then another town. Then another town. He met a nice lady who worked at the circus, because she was always changing towns too. Then they fell in love real hard.
"I like you you're cute." she'd say to him.
"Aw that's nice of you."
The circus lady had a great idea of keeping him in a traveling circus cage where they usually keep animals. She was very ugly. Also she was very organized so she kept track when full moons would happen. So Barry didn't have to stay in the cage all the time.
Turning into a Changebeast was like the equivalent of Barry having a period menstrual cycle. When he got older he went through manopaws and it stopped. They were finally happy. They had a lovely son. Barry and the Circus Lady didn't realize that the Changebeast Syndrome was hereditary and was passed down to their ugly child. One day the son turned into a Changebeast and massacred them all.