platy late bloomer

[NorthernStar]

I bought this Koi platy a few weeks ago. My lfs told me it was a female. When I took it home, my smaller Mickey Mouse male platy had no interest. So another week later I bought a green female swordtail. Now my Koi keeps following around the female. I took a closer look at the Koi and realized that "she" was a "he". The anal fin now looks more like a gonopodium. Am I right?

 

[coralbandit]

Your koi is a female as it is a swordtail not a platy...They cross breed no problem....
The anal fin is how you tell live bearer sex...The koi has a fan shaped fin ,hence female...That and the lack of sword would be a clear enough indicator for me....