This story had a good premise to it but there were several issues that really made this hard to read. It would really help your stories if you could have someone proof read your grammar for you. You over used your commas way too much and your sentences were short and blocky. Your use of English was good but it was kind of clear from your sentence structure that English might not be your first language. One big thing to work on would be your pacing in your story. 

It was a bit confusing that sometimes your would use quotes and sometimes you wouldn't. It isn't necessary to use parentheses as well. Also, you don't have to add a super long line of dashes and write in "FLASHBACK." You could just put a couple dashes and then start your next paragraph with, "It happened two months ago when I first met her..." Or something like that.

Also the motivations for the main character weren't clear. He was avoiding the Nintails because he was upset that she tried to rape him in his sleep one night? But then when she finds him again he just gives into embracing her without a fight? Then all of a sudden from having sex with her one time he is in love with her? It doesn't make sense. 

I did like that you tried showing an appropriate lifestyle for the Pokemon being feral. That was a nice touch. But it felt like it was only a footnote that you didn't explore very much. Your descriptive detail felt more like you were just listing information off rather than painting the scene for us to explore along with your character. 

It seems like you have some really good ideas for what it is you want to write about but you need some work on your execution. Again, work on your pacing and that alone will help a lot. Also explore writing out a scene without adding so many commas. And then study descriptive narration to help bring your scenes to life.