Aw, I like the unicorn XP
I love how this story jumps into the adventure right off the bat. That's a good way to grab people's attention. Is Rio going to run into the unicorn's captors? Already, it seems she might be in danger.
This was a good first chapter, even if it was short. The dead fish was a nice touch.
The only complaint I have is the lack of an established setting. The reader is left rather uninformed as to what's going on so the excitement of discovering this mythical creature (unicorn) is overwhelming in a way. Maybe you could explain a little more who Rio is, where they are, etc.
Please update soon. I'm compelled to find out what happens next.